Showing papers in "Journal of Computational Chemistry in 1996"

Merck molecular force field. I. Basis, form, scope, parameterization, and performance of MMFF94

Abstract: This article introduces MMFF94, the initial published version of the Merck molecular force field (MMFF). It describes the objectives set for MMFF, the form it takes, and the range of systems to which it applies. This study also outlines the methodology employed in parameterizing MMFF94 and summarizes its performance in reproducing computational and experimental data. Though similar to MM3 in some respects, MMFF94 differs in ways intended to facilitate application to condensed-phase processes in molecular-dynamics simulations. Indeed, MMFF94 seeks to achieve MM3-like accuracy for small molecules in a combined “organic/protein” force field that is equally applicable to proteins and other systems of biological significance. A second distinguishing feature is that the core portion of MMFF94 has primarily been derived from high-quality computational data—ca. 500 molecular structures optimized at the HF/6-31G* level, 475 structures optimized at the MP2/6-31G* level, 380 MP2/6-31G* structures evaluated at a defined approximation to the MP4SDQ/TZP level, and 1450 structures partly derived from MP2/6-31G* geometries and evaluated at the MP2/TZP level. A third distinguishing feature is that MMFF94 has been parameterized for a wide variety of chemical systems of interest to organic and medicial chemists, including many that feature frequently occurring combinations of functional groups for which little, if any, useful experimental data are available. The methodology used in parameterizing MMFF94 represents a fourth distinguishing feature. Rather than using the common “functional group” approach, nearly all MMFF parameters have been determined in a mutually consistent fashion from the full set of available computational data. MMFF94 reproduces the computational data used in its parameterization very well. In addition, MMFF94 reproduces experimental bond lengths (0.014 A root mean square [rms]), bond angles (1.2° rms), vibrational frequencies (61 cm−1 rms), conformational energies (0.38 kcal/mol/rms), and rotational barriers (0.39 kcal/mol rms) very nearly as well as does MM3 for comparable systems. MMFF94 also describes intermolecular interactions in hydrogen-bonded systems in a way that closely parallels that given by the highly regarded OPLS force field. © 1996 John Wiley & Sons, Inc.

Using redundant internal coordinates to optimize equilibrium geometries and transition states

Abstract: A redundant internal coordinate system for optimizing molecular geometries is constructed from all bonds, all valence angles between bonded atoms, and all dihedral angles between bonded atoms. Redundancies are removed by using the generalized inverse of the G matrix; constraints can be added by using an appropriate projector. For minimizations, redundant internal coordinates provide substantial improvements in optimization efficiency over Cartesian and nonredundant internal coordinates, especially for flexible and polycyclic systems. Transition structure searches are also improved when redundant coordinates are used and when the initial steps are guided by the quadratic synchronous transit approach. © 1996 by John Wiley & Sons, Inc.

The role of databases in support of computational chemistry calculations

Abstract: A role for electronic structure databases in assisting users of quantum chemistry applications select better model parameters is discussed in light of experiences gained from a software prototype known as the Computational Chemistry Input Assistant (CCIA). It is argued that the ready availability of information pertaining to the applications and theoretical models can substantially increase the likelihood of novice users obtaining the desired accuracy from their calculations while simultaneously making better use of computer resources. Expert users, who find themselves contemplating studies in new areas of research, may also benefit from the proposed tools. For maximum impact, this assistance should be provided while users are actively engaged in preparing calculations. © 1996 by John Wiley & Sons, Inc.

Merck molecular force field. II. MMFF94 van der Waals and electrostatic parameters for intermolecular interactions

Abstract: This article defines the parameterization and performance of MMFF94 for intermolecular interactions. It specifies the novel “buffered” functional forms used for treating van der Waals (vdW) and electrostatic interactions, and describes the use of : (1) high quality ab initio data to parameterize vdW interactions involving aliphatic hydrogens; and (2) HF/6-31G* calculations on hydrogen-bonded complexes to parameterize nonbonded interactions in polar systems. Comparisons show that appropriate trends in the HF/6-31G* data are well reproduced by MMFF94 and that intermolecular interaction energies and geometries closely parallel those given by the highly regarded OPLS force field. A proper balance between solvent–solvent, solvent–solute, and solute–solute interactions, critically important for prospective success in aqueous simulations, thus appears to be attained. Comparison of MMFF94, OPLS, CHELPG electrostatic potential fit, QEq, Gasteiger, and Abraham charges for 20 small molecules and ions also shows the close correspondence between MMFF94 and OPLS. As do OPLS and all current, widely used force fields, MMFF94 employs “effective pair potentials” which incorporate in an averaged way the increases in polarity which occur in high dielectric media. Some limitations of this approach are discussed and suggestions for possible enhancements to MMFF94's functional form are noted. © 1996 John Wiley & Sons, Inc.

Merck molecular force field. IV. conformational energies and geometries for MMFF94

Abstract: This article describes the parameterization and performance of MMFF94 for conformational energies, rotational barriers, and equilibrium torsion angles. It describes the derivation of the torsion parameters from high-quality computational data and characterizes MMFF94's ability to reproduce both computational and experimental data, the latter particularly in relation to MM3. The computational data included: (i) ∼ 250 comparisons of conformational energy based on “MP4SDQ/TZP” calculations (triple-zeta plus polarization calculations at a defined approximation to the highly correlated MP4SDQ level) at MP2/6-31G* geometries; and (ii) ∼ 1200 MP2/TZP comparisons of “torsion profile” structures at geometries derived from MP2/6-31G* geometries. The torsion parameters were derived in restrained least-squares fits that used the complete set of available computational data, thereby ensuring that a fully optimal set of parameters would be obtained. The final parameters reproduce the “MP4SDQ/TZP” and MP2/TZP computational data with root mean square (rms) deviations of 0.31 and 0.50 kcal/mol, respectively. In addition, MMFF94 reproduces a set of 37 experimental gas-phase and solution conformational energies, enthalpies, and free energies with a rms deviation of 0.38 kcal/mol; for comparison, the “MP4SDQ/TZP” calculations and MM3 each gives a rms deviation of 0.37 kcal/mol. Furthermore, MMFF94 reproduces 28 experimentally determined rotational barriers with a rms deviation of 0.39 kcal/mol. Given the diverse nature of the experimental conformational energies and rotational barriers and the clear indications of experimental error in some cases, the MMFF94 results appear excellent. Nevertheless, MMFF94 encounters somewhat greater difficulty in handling multifunctional compounds that place highly polar functional groups in close proximity, probably because it, like other commonly used force fields, too greatly simplifies the description of electrostatic interactions. Some suggestions for enhancements to MMFF94's functional form are discussed. © 1996 John Wiley & Sons, Inc.

Merck Molecular Force Field. III. Molecular Geometries and Vibrational Frequencies for MMFF94

Abstract: This article describes the parameterization and performance of MMFF94 for molecular geometries and deformations. It defines the form used for the valence-coordinate terms that represent variations in bond lengths and angles, and it describes the derivation of quadratic force constants from HF/6-31G* data and the derivation of reference bond lengths and angles from fits to MP2/6-31G*-optimized geometries. Comparisons offered show that MMFF94 accurately reproduces the computational data used in its parameterization and demonstrate that its derivation from such data simultaneously confers the ability to reproduce experiment. In particular, MMFF94 reproduces experimentally determined bond lengths and angles for 30 organic molecules with root mean square (rms) deviations of 0.014 A and 1.2°, respectively. MM3 reproduces bond angles to the same accuracy, but reproduces experimental bond lengths more accurately, in part because it was fit directly to thermally averaged experimental bond lengths; MMFF94, in contrast, was fit to (usually shorter) energy-minimum values, as is proper for an anharmonic force field intended for use in molecular-dynamics simulations. The comparisons also show that UFF and a recent version of CHARMm (QUANTA 3.3 parameterization) are less accurate for molecular geometries than either MMFF94 or MM3. For vibrational frequencies, MMFF94 and MM3 give comparable overall rms deviations versus experiment of 61 cm−1 and 57 cm−1, respectively, for 15 small, mostly organic molecules. In a number of instances, MM3's derivation employed observed frequencies that differ substantially—by nearly 400 cm−1 in one case—from other published frequencies which had themselves been confirmed theoretically by good-quality ab initio calculations. Overall, the comparisons to experimental geometries and vibrational frequencies demonstrate that MMFF94 achieves MM3-like accuracy for organic systems for which MM3 has been parameterized. Because MMFF94 is derived mainly from computational data, however, it has been possible to parameterize MMFF94 with equal rigor for a wide variety of additional systems for which little or no useful experimental data exist. Equally good performance can be expected for such systems. © John Wiley & Sons, Inc.

Merck molecular force field. V. Extension of MMFF94 using experimental data, additional computational data, and empirical rules

Abstract: This article describes the extension of the Merck Molecular Force Field (MMFF94) to a much broader range of organic systems. It also describes a preliminary parameterization of MMFF94 for the hydronium and hydroxide ions and for various halide, alkalai, and alkalai earth ions as well as for such “protein” metals as Zn2+, Ca2+, Cu2+, Cu+, Fe2+, and Fe3+. The extension employed computational data on charge distributions, molecular geometries, and conformational energies for a series of oxysulfur (particularly sulfonamide) and oxyphosphorous compounds and for a diverse set of small molecules and ions not covered in the core parameterization. It also employed experimental data for approximately 2800 good-quality structures extracted from the Cambridge Structural Database (CSD). Some of the additional computational data were used to extend the explicit parameterization of electrostatic interactions and to more widely define a useful additive approximation for the “bond polarity” parameters (bond charge increments) used in MMFF94. Both the experimental and computational data served to define reference bond lengths and angles that the extended force field uses in conjunction with force constants obtained from carefully calibrated empirical rules. The extended torsion parameters consist partly of explicit parameters derived to reproduce MP2/6-31G* conformational energies and partly of “default parameters” provided by empirical rules patterned after those used in DREIDING and UFF but calibrated, where possible, against computationally derived MMFF94 torsion parameters. Comparisons to experimental data show that MMFF94 reproduces crystallographic bond lengths and bond angles with relatively modest root mean square (rms) deviations of approx. 0.02 A and 2°, respectively. © John Wiley & Sons, Inc.

A fast method of molecular shape comparison: A simple application of a Gaussian description of molecular shape

Abstract: A Gaussian description of molecular shape is used to compare the shapes of two molecules by analytically optimizing their volume intersection. The method is applied to predict the relative orientation of ligand series binding to the proteins, thrombin, HIV protease, and thermolysin. The method is also used to quantify the degree of chirality of asymmetric molecules and to investigate the chirality of biphenyl and the amino acids. The shape comparison method uses the newly described shape multipoles that can also be used to describe the inherent shape of molecules. Some results of calculated shape quadrupoles are given for the ligands used in this work. © 1996 by John Wiley & Sons, Inc.

Basis sets for transition metals: Optimized outer p functions.

Abstract: Although the (n + 1)p orbital is unoccupied in transition-metal ground-state configurations which are all nd(x) (n + 1)s(y) , these (n + 1)p functions play a crucial role in the structure of transition metal complexes. As we show here, the usual solution, adding one or more diffuse functions, can be insufficient to create an orbital of the correct energy. The major problem appears to be due to the incorrect placement of the (n + 1)p orbital's node. Even splitting the most diffuse component of the np orbital and adding a second diffuse function does not completely solve this problem. Although one can usually solve this deficiency by further uncontracting of the np function, here we offer a set of properly optimized (n + 1)p functions that offer a more compact and satisfactory solution to the proper placements of the node. We show an example of the common deficiencies seen in typical basis sets, including standard basis sets in GAUSSIAN94, and show that the new optimized (n + 1)p function performs well compared to a fully uncontracted basis set. © 1996 by John Wiley & Sons, Inc.

An improved force field (MM4) for saturated hydrocarbons

Abstract: A new force field has been developed for alkanes and cycloalkanes, excluding small rings, to improve the calculation of vibrational frequencies, rotational barriers, and numerous relatively small errors that were observed to result from the use of the MM3 force field. © 1996 by John Wiley & Sons, Inc.

Spherical symmetric diffusion problem

Abstract: We introduce a general and versatile MS Windows application for solving the spherically symmetric diffusion problem, involving up to two coupled spherically symmetric Smoluchowski equations. The application is based on a modular, configurable, user-friendly graphical interface, in which input parameters are introduced through a graphical representation of the system of partial differential equations and output attributes can be obtained graphically during propagation. The numerical algorithm consists of finite differencing in space and Chebyshev propagation in time; it includes an implementation of virtual gridding, which enhances the accuracy of calculating boundary conditions and steep potentials. The program has b een checked against a wide collection of analytical solutions and applied to an experimentally open problem in excited-state proton-transfer to solvent. © 1996 by John Wiley & Sons, Inc.

Optimization of the Lennard‐Jones parameters for a combined ab initio quantum mechanical and molecular mechanical potential using the 3‐21G basis set

Abstract: A combined ab initio quantum mechanical and molecular mechanical (AI-QM/MM) potential for use in molecular modeling and simulation has been described. In this article, we summarize a procedure for deriving the empirical parameters embedded in a combined QM/MM model and suggest a set of Lennard-Jones parameters for the combined ab initio 3-21G and MM OPLS-TIP3P (AI-3/MM) potential. Interaction energies and geometrical parameters predicted with the AI-3/MM model for over 80 hydrogen-bonded complexes of organic compounds with water were found to be in good accord with ab initio 6-31G(d) results. We anticipate that the AI-3/MM potential should be reasonable for use in condensed phase simulations. © 1996 by John Wiley & Sons, Inc.

Computing ionization states of proteins with a detailed charge model

Abstract: A convenient computational approach for the calculation of the p Kas of ionizable groups in a protein is described. The method uses detailed models of the charges in both the neutral and ionized form of each ionizable group. A full derivation of the theoretical framework is presented, as are details of its implementation in the UHBD program. Application to four proteins whose crystal structures are known shows that the detailed charge model improves agreement with experimentally determined pKas when a low protein dielectric constant is assumed, relative to the results with a simpler single-site ionization model. It is also found that use of the detailed charge model increases the sensitivity of the computed pKas to the details of proton placement. © 1996 by John Wiley & Sons, Inc.

Charges fit to electrostatic potentials. II. Can atomic charges be unambiguously fit to electrostatic potentials

Abstract: The present work examines the conditioning of the least-squares matrix for obtaining potential derived charges and presents a modification of the CHELP method for fitting atomic charges to electrostatic potentials. Results from singular value decompositions (SVDs) of the least-squares matrices show that, in general, the least-squares matrix for this fitting problem will be rank deficient. Thus, statistically valid charges cannot be assigned to all the atoms in a given molecule. We find also that, contrary to popular notions, increasing the point density of the fit has little or no influence on the rank of the problem. Improvement in the rank can best be achieved by selecting points closer to the molecular surface. Basis set has, as expected, no effect on the number of charges that can be assigned. Finally, a well-defined, computationally efficient algorithm (CHELP-SVD) is presented for determining the rank of the least-squares matrix in potential-derived charge fitting schemes, selecting the appropriate subset of atoms to which charges can be assigned based on that rank estimate, and then refitting the selected set of charges. 0 1996 by John Wiley & Sons, Inc.

Calculation of absolute binding free energies for charged ligands and effects of long‐range electrostatic interactions

Abstract: Calculation of Absolute Binding Free Energies for Charged Ligands and the Effects of Long-Range Electrostatic Interactions

Base stacking in cytosine dimer. A comparison of correlated ab initio calculations with three empirical potential models and density functional theory calculations

Abstract: Ab initio MP2/6-31G* interaction energies were calculated for more than 80 geometries of stacked cytosine dimer. Diffuse polarization functions were used to properly cover the dispersion energy. The results of ab initio calculations were compared with those obtained from three electrostatic empirical potential models, constructed as the sum of a Lennard-Jones potential (covering dispersion and repulsion contributions) and the electrostatic term. Point charges and point multipoles of the electrostatic term were also obtained at the MP2/6-31G* level of theory. The point charge MEP model (atomic charges derived from molecular electrostatic potential) satisfactorily reproduced the ab initio data. Addition of π-charges localized below and above the cytosine plane did not affect the calculated energies. The model employing the distributed multipole analysis gave worse agreement with the ab initio data than the MEP approach. The MP2 MEP charges were also derived using larger sets of atomic orbitals: cc-pVDZ, 6-311 + G(2d, p), and aug-cc-pVDZ. Differences between interaction energies calculated using these three sets of point charges and the MP2/6-31G* charges were smaller than 0.8 kcal/mol. The correlated ab initio calculations were also compared with the density functional theory (DFT) method. DFT calculations well reproduced the electrostatic part of interaction energy. They also covered some nonelectrostatic short-range effects which were not reproduced by the empirical potentials. The DFT method does not include the dispersion energy. This energy, approximated by an empirical term, was therefore added to the DFT interaction energy. The resulting interaction energy exhibited an artifact secondary minimum for a 3.9-4.0 vertical separation of bases. This defect is inherent in the DFT functionals, because it is not observed for the Hartree-Fock + dispersion interaction energy.© 1996 John Wiley & Sons, Inc.

A comparison of conformational energies calculated by several molecular mechanics methods

Abstract: Several commonly used molecular mechanics force fields have been tested for accuracy in conformational energy calculations. Differences in performance between the force fields are discussed for different classes of structures. MMFF93 and force fields based on the MM2 or MM3 functional form are found to perform significantly better than other force fields in the test, with average conformational energy errors around 0.5 kcal/mol. CFF91 also reaches this accuracy for the subset in which fully determined parameters are used, but it doubles the overall error due to use of estimated parameters. Harmonic force fields generally have average errors exceeding 1 kcal/mol. Factors influencing accuracy are identified and discussed. © 1996 by John Wiley & Son s, Inc.

A new parallel method for molecular dynamics simulation of macromolecular systems

Abstract: Short-range molecular dynamics simulations of molecular systems are commonly parallelized by replicated-data methods, in which each processor stores a copy of all atom positions. This enables computation of bonded 2-, 3-, and 4-body forces within the molecular topology to be partitioned among processors straightforwardly. A drawback to such methods is that the interprocessor communication scales as N (the number of atoms) independent of P (the number of processors). Thus, their parallel efficiency falls off rapidly when large numbers of processors are used. In this article a new parallel method for simulating macromolecular or small-molecule systems is presented, called force-decomposition. Its memory and communication costs scale as N/√P, allowing larger problems to be run faster on greater numbers of processors. Like replicated-data techniques, and in contrast to spatial-decomposition approaches, the new method can be simply load balanced and performs well even for irregular simulation geometries. The implementation of the algorithm in a prototypical macromolecular simulation code ParBond is also discussed. On a 1024-processor Intel Paragon, ParBond runs a standard benchmark simulation of solvated myoglobin with a parallel efficiency of 61% and at 40 times the speed of a vectorized version of CHARMM running on a single Cray Y-MP processor. © 1996 by John Wiley & Sons, Inc.

Potential derived charges using a geodesic point selection scheme

Abstract: Potential derived (PD) atomic charges, obtained by fitting to molecular electrostatic potentials, are widely used in molecular modeling and simulation calculations These charges are known to depend on the sample of points chosen for the fit, on the particular point selection algorithm, on molecular translations and rotations in many instances, and even on molecular conformation Following a critique of currently available methods, a novel point selection scheme is described which results in a highly isotropic array of points located on a series of fused-sphere van der Waals surfaces The pattern of points is based on tesselations of the icosahedron, and these are discussed in some detail along with their connection with virus morphology, geodesic domes, and symmetric fullerene structures Using methanol as a test case, it is shown that the new method leads to PD charges which are independent of translation and display minimal rotational dependence, and are hence far better suited to the determination of PD charges from electrostatic potentials obtained from both theory and experimental X-ray diffraction data The conformation dependence of the newly derived PD charges for alanyl dipeptide is found to be substantially less than obtained earlier by Williams [Biopolymers29, 1367 (1990)] © 1996 by John Wiley & Sons, Inc

A coupled density functional‐molecular mechanics Monte Carlo simulation method: The water molecule in liquid water

Abstract: A theoretical model to investigate chemical processes in solution is described. It is based on the use of a coupled density functional/molecular mechanics Hamiltonian. The most interesting feature of the method is that it allows a detailed study of the solute's electronic distribution and of its fluctuations. We present the results for isothermal‐isobaric constant‐NPT Monte Carlo simulation of a water molecule in liquid water. The quantum subsystem is described using a double‐zeta quality basis set with polarization orbitals and nonlocal exchange‐correlation corrections. The classical system is constituted by 128 classical TIP3P or Simple Point Charge (SPC) water molecules. The atom‐atom radial distribution functions present a good agreement with the experimental curves. Differences with respect to the classical simulation are discussed. The instantaneous and the averaged polarization of the quantum molecule are also analyzed. © 1996 by John Wiley & Sons, Inc.

Force field parameters for carbohydrates

Abstract: A new set of force field parameters for carbohydrates is reported. The parameter set is based on the CHARMM22 force field of Karplus and co-workers. The parameterization is based on newly performed high-level ab initio calculations [MP2/6-311 + G (2d, 2p)/ /6-31G**] of fragment molecules. A good agreement of the modified force field and ab initio data is achieved, which is demonstrated with a variety of molecules. © 1996 by John Wiley & Sons, Inc.

Extension of MST/SCRF method to organic solvents: Ab initio and semiempirical parametrization for neutral solutes in CCl4

Abstract: The self-consistent reaction field (SCRF) method proposed by Miertus, Scrocco, and Tomasi (MST) was extended to solutions of neutral solutes in CCl4. A detailed parametrization of the solute/solvent interface and of the “hardness” atomic parameters determining the van der Waals interactions was performed from comparison with experimental data and Monte Carlo simulations. The parametrization was carried out at both ab initio (6-31G*) and semiempirical (MNDO, AM1, PM3) levels. The MST/SCRF optimized versions provide accurate estimates of the free energy of solvation in CCl4 for the series of molecules studied. Furthermore, a precise description of the solvent effect on different chemical processes in CCl4 solution supports the reliability of the parametrization. © 1996 by John Wiley & Sons, Inc.

Method for free‐energy calculations using iterative techniques

Abstract: We present here a new iterative technique for reliable estimation of multidimensional free energy and potential of mean force (PMF) values by computer simulation. This method is an extension of the weighted histogram analysis method [S. Kumar et al., J. Comput. Chem.,13, 1011, (1992)]. We have tested the technique by generating free-energy-based Ramachandran plots and by computing the PMF values for end-to-end distances for several polypeptides using the ECEPP/2 and AMBER force fields. © 1996 by John Wiley & Sons, Inc.

Analytical first derivatives of molecular surfaces with respect to nuclear coordinates

Abstract: We present analytical expressions for the first derivatives of area and other geometrical quantities of polygonal tesserae defined on molecular surfaces. This is a necessary step in the calculation of free energy derivatives with respect to nuclear coordinates for molecular solutes, in the framework of the polarizable continuum method. An application to solute‐solvent dispersion energy derivatives is presented. © 1996 by John Wiley & Sons, Inc.

Molecular Dynamics Simulation of a Polymer Chain in Solution by Collisional Dynamics Method

Abstract: This article describes the collisional dynamics (CD) method adapted for molecules with geometrical constraints within a description using Cartesian coordinates for the atoms. In the CD method, stochastic collisions with virtual particles are included in usual molecular dynamics simulations to couple the considered polymer molecule to a solvent. The actual presence of the solvent is not explicitly included in the simulation. The results of CD simulations of a polymer chain immersed in the time-dependent elongational flow field are presented. The influence of nonbonded interactions on the coil-stretch transition of the chain occurring in the flow is discussed. Sons, Inc. 0 1996 John Wiley &

Semiempirical treatment of electrostatic potentials and partial charges in combined quantum mechanical and molecular mechanical approaches

Abstract: A semiempirical treatment of electrostatic potentials and partial charges is presented. These are the basic components needed for the evaluation of electrostatic interaction energies in combined quantum mechanical and molecular mechanical approaches. The procedure to compute electrostatic potentials uses AM1 and MNDO wave functions and is based on one previously suggested by Ford and Wang. It retains the NDDO approximation and is thus both easy to implement and computationally efficient. Partial atomic charges are derived from a semiempirical charge equilibration model, which is based on the principle of electronegativity equalization. Large sets of ab initio restricted Hartee-Fock (RHF/6-31G*) reference data have been used to calibrate the semiempirical models. Applying the final parameters (C, H, N, O), the ab initio electrostatic potentials are reproduced with an average accuracy of 20% (AM1) and 25% (MNDO), respectively, and the ab initio potential derived charges normally to within 0.1 e. In most cases our parameterized models are more accurate than the much more expensive quasi ab initio techniques, which employ deorthogonalized semiempirical wave functions and have generally been preferred in previous applications. © 1996 John Wiley & Sons, Inc.

Comparative binding energetics of Mg2+, Ca2+, Zn2+, and Cd2+ to biologically relevant ligands: Combined ab initio SCF supermolecule and molecular mechanics investigation

Abstract: A combined ab initio SCF supermolecule and molecular mechanics investigation is carried out on the binding energetics of the divalent cations Mg2+, Ca2+, Zn2+, and Cd2+ to a series of the most common ligand functional groups found in biomolecules. The SCF binding energy components are resolved using the restricted variational space method.1 The results show that the SIBFA molecular mechanics (SMM) procedure2 reproduces the ab initio binding energies and total energy variations as a function of intermolecular variables. The model also reproduces the selectivity energetics for exchange reactions. Thus, the SMM procedure can be used without reparametrization to describe the coordination energetics of complex molecules including those subject to coordination changes. The energetic properties of divalent cation‐hexahydrate complexes are compared as examples of a complete, realistic coordination system. The hexahydrates exhibit strong nonadditive effects typical of dication coordination. Nevertheless, these energetics are satisfactorily reproduced by the SMM procedure. © 1996 John Wiley & Sons, Inc.

Finite-difference solution of the Poisson-Boltzmann equation: Complete elimination of self-energy.

Abstract: A new procedure to solve the Poisson-Boltzmann equation is proposed and shown to be efficient. The electrostatic potential due to the reaction field is calculated directly. Self-interactions among the charges are completely eliminated. Therefore, the reference calculation to cancel out the self-energy is not needed. © 1996 by John Wiley & Sons, Inc.

The 1:1 glycine zwitterion‐water complex: An ab initio electronic structure study

Abstract: More than a dozen stationary points on the potential energy surface for the 1:1 glycine zwitterion—water complex have been investigated at Hartree-Fock or MP2 levels of theory with basis sets ranging from split valence (4-31G) to split valence plus polarization and diffuse function (6–31 + + G**) quality. Only one true minimum (GLYZWM, C1 symmetry) could be located on the potential energy surface. GLYZWM features a bridged water molecule acting as both a hydrogen bond acceptor and donor with the NH3− and CO2− units of the glycine zwitterion. The total hydrogen bond energy in GLYZWM is computed as 16 kcal/mol (MP2/6–31 ++ G** // 6–31 ++ G**, including corrections for basis set superpositions errors). The computed vibrational frequencies and normal mode forms of the GLYZWM complex resemble in many cases experimental assignments made for the glycine zwitterion in bulk water on the basis of Raman spectroscopy. © 1996 by John Wiley & Sons, Inc.

Molecular mechanics (MM4) calculations on conjugated hydrocarbons

Abstract: The MM4 force field has been extended to conjugated hydrocarbon systems. It retains most of the formalism and computational schemes that were present in MM3. Several cross-terms have been added in MM4 that were not present in MM3, mainly to improve vibrational frequencies, but also to improve structures and energies. Additional bond order dependence equations have been included. About 80 molecules have been examined, some in multiple conformations, and others with more complete experimental profiles. Conformational energy differences/barriers are generally fit to within 0.5 kcal/mol unless they are very large. The MM4 method for calculating heats of formation has been modified to include vibrational as well as steric energy contributions. The heats of formation for 35 alkenes and 57 conjugated hydrocarbons were determined to have an overall root mean square (rms) deviation of 0.68 kcal/mol from experiment (0.47 kcal/mol weighted rms). The vibrational frequency rms error for eight conjugated hydrocarbons is 31 cm1. Geometries are fit for the most part to within the following ranges: 0.004 A for bond lengths, 1° for bond angles, 4° for torsion angles, and 0.5% for moments of inertia (r2). © 1996 by John Wiley & Sons, Inc.