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Showing papers in "Journal of Computational Chemistry in 1987"


Journal ArticleDOI
TL;DR: In this paper, a new algorithm for fitting atomic charges to molecular electrostatic potentials is presented, which is non-iterative and rapid compared to previous work. But this method is not suitable for a large number of atoms and anions, and the effects of using experimental and optimized geometries are explored.
Abstract: A new algorithm for fitting atomic charges to molecular electrostatic potentials is presented. This method is non-iterative and rapid compared to previous work. Results from a variety of gaussian basis sets, including STO-3G, 3-21G and 6-31G*, are presented. Charges for a representative collection of molecules, comprising both first and second row atoms and anions are tabulated. The effects of using experimental and optimized geometries are explored. Charges derived from these fits are found to adequately reproduce SCF dipole moments. A small split valence representation, 3-21G, appears to yield consistently good results in a reasonable amount of time.

974 citations


Journal ArticleDOI
TL;DR: In this paper, a variant of the Truncated Newton nonlinear optimization procedure is proposed for potential energy minimization of large molecular systems, which shows particular promise for large molecular system.
Abstract: Techniques from numerical analysis and crystallographic refinement have been combined to produce a variant of the Truncated Newton nonlinear optimization procedure. The new algorithm shows particular promise for potential energy minimization of large molecular systems. Usual implementations of Newton's method require storage space proportional to the number of atoms squared (i.e., O(N2)) and computer time of O(N3). Our suggested implementation of the Truncated Newton technique requires storage of less than O(N1.5) and CPU time of less than O(N2) for structures containing several hundred to a few thousand atoms. The algorithm exhibits quadratic convergence near the minimum and is also very tolerant of poor initial structures. A comparison with existing optimization procedures is detailed for cyclohexane, arachidonic acid, and the small protein crambin. In particular, a structure for crambin (662 atoms) has been refined to an RMS gradient of 3.6 × 10−6 kcal/mol/A per atom on the MM2 potential energy surface. Several suggestions are made which may lead to further improvement of the new method.

858 citations


Journal ArticleDOI
TL;DR: In this article, an algorithm for a finer description of cavities in continuous media and for a more efficient selection of sampling points on the cavity surface is described, with applications to the evaluation of solute surface and volume and to the calculation of the solute-solvent electrostatic interaction energy, as well as of the cavitation energy.
Abstract: Algorithms for a finer description of cavities in continuous media and for a more efficient selection of sampling points on the cavity surface are described. Applications to the evaluation of solute surface and volume and to the calculation of the solute-solvent electrostatic interaction energy, as well as of the cavitation energy are shown as examples.

442 citations


Journal ArticleDOI
TL;DR: In this paper, a series of efficient split-valence basis sets for first-row transition metals, termed 3-21G, has been constructed based on previously-formulated minimal expansions of Huzinaga, in which each atomic orbital has been represented by a sum of three gaussians.
Abstract: A series of efficient split-valence basis sets for first-row transition metals, termed 3-21G, has been constructed based on previously-formulated minimal expansions of Huzinaga, in which each atomic orbital has been represented by a sum of three gaussians. The original Huzinaga expansions for s- and p-type orbitals (except those for 1s) have been fit by least squares to new three-gaussian combinations in which the two sets of orbitals (of the same n quantum number) share gaussian exponents. The Huzinaga three-gaussian expansions for ls and 3d atomic orbitals have been employed without alteration. The valence description of the 3-21G basis sets comprises 3d-, 4s- and 4p-type functions, each of which has been split into two- and one-gaussian parts. 4p functions, while not populated in the ground state of the free atoms, are believed to be important to the description of the bonding in molecules. The performance of the 3-21G basis sets is examined with regard to the calculation of equilibrium geometries and normalmode vibrational frequencies for a variety of inorganic and organometallic compounds containing first-row transition metals. Calculated equilibrium structures, while generally superior to those obtained at STO-3G, are not as good as those for compounds containing main-group elements only. The calculations generally underestimate the lengths of double bonds between transition metals and main-group elements, and overestimate the lengths of single linkages. Calculated normal-mode vibrational frequencies for metal-containing systems are less uniform than in those for main-group compounds.

429 citations


Journal ArticleDOI
TL;DR: The molecular mechanics (MMP2) program and procedures for the treatment of conjugated hydrocarbons, and some of the results which they can achieve are described in this article.
Abstract: The molecular mechanics (MMP2) program and procedures for the treatment of conjugated hydrocarbons, and some of the results which they can achieve are described. The program is an updated version of the similar MMP1 program, but contains some differences. It is based on an SCF π system calculation, rather than on the VESCF method used earlier. All parameters are compatible with those in the MM2 program. Hence it is possible to calculate heats of formation, resonance energies, and structures for conjugated hydrocarbons in a way that is consistent with the calculations on non-conjugated molecules. The overall results as far as structure and energy are somewhat better than they were with the MMP1 program.

206 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of the addition of diffuse functions to molecules with second-row elements is investigated. But the results show that diffuse functions have little effect on the geometries and vibrational frequencies of neutral molecules and significant changes for anions.
Abstract: The energetic effects of the addition of diffuse functions to molecules with second-row elements are much less dramatic than those for their first-row counterparts. Although diffuse functions on second-row elements have little effect on the geometries and vibrational frequencies of neutral molecules, significant changes are found for anions. While the largest basis set, 6-31 + G*, generally performs best, the results at 3-21 + G* are comparable, and this basis can be recommended for practical applications.

189 citations


Journal ArticleDOI
TL;DR: In this paper, a vector processor is used to calculate the electron density function in three dimensions, such as the density itself, the surrounding electrostatic potential, ∇ρ, and ∇2ρ.
Abstract: Accurate and efficient integration of the electron density function over arbitrary regions has been previously achieved by exploiting a separation of variables. Recently, a computer program has been written that calculates ρ, ρ, and ∇2ρ in an expeditious fashion, taking advantage of the separation of variables in the electron density function. Accurate integrations of ∇2ρ over arbitrary regions can also be accomplished. The structure of the program is suited especially to vector processors. As a result of the efficiencies of these programs, functions of the electron density, such as the density itself, the surrounding electrostatic potential, ∇ρ, and ∇2ρ have been calculated in three dimensions. Results of calculations for nitrated cubanes are presented illustrating how the effects of the nitro groups are manifested in the electron density and associated properties.

143 citations


Journal ArticleDOI
TL;DR: The evolution of the CIPSI method, with the latest modifications recently implemented in our laboratory, is described in this paper, where a new version, based on a diagrammatic technique, is presented.
Abstract: The evolution of the CIPSI method, with the latest modifications recently implemented in our laboratory, is described. A new version, based on a diagrammatic technique, is presented. Test calculations which have been run on water, ethylene, and transacrolein, show that the new method is a powerful tool for the study of medium-size molecular systems.

130 citations


Journal ArticleDOI
TL;DR: A numerical algorithm for locating both minima and transition states designed for use in the ab initio program package GAUSSIAN 82 is presented and is effectively the numerical version of an analytical algorithm (OPT = EF) previously published in this journal.
Abstract: A numerical algorithm for locating both minima and transition states designed for use in the ab initio program package GAUSSIAN 82 is presented It is based on the RFO method of Simons and coworkers and is effectively the numerical version of an analytical algorithm (OPT = EF) previously published in this journal The algorithm is designed to make maximum use of external second derivative information obtained from prior optimizations at lower levels of theory It can be used with any wave function for which an energy can be calculated and is about two to three times faster than the default DFP algorithm (OPT = FP) supplied with GAUSSIAN 82

125 citations


Journal ArticleDOI
TL;DR: In this article, a two-parameter family of various homology groups and cohomology groups of charge density contour surfaces is proposed for a detailed characterization of the shapes of electronic charge distributions of general, asymmetric molecules.
Abstract: A group theoretical framework is proposed for a detailed characterization of the shapes of electronic charge distributions of general, asymmetric molecules. The proposed shape groups are the homology and cohomology groups of charge density contour surfaces. These shape groups depend on two real parameters, the charge density value a for the contour and a curvature parameter b. The two-parameter family of various homology groups and cohomology groups of charge density contour surfaces is independent of the symmetry properties of the molecules and gives a concise description of the dominant shape characteristics. For any fixed parameter value b these groups may change only at specific charge density values, characteristic to the given molecule. On the other hand, for a fixed-charge density contour the group changes induced by a change in the curvature parameter b provide a description of the fine details of the shape of the electron density. The changes in the structure of these groups follow strict algebraic relations, that provide a quantitative measure for shape-similarity between various molecules. The two-parameter shape group method is an extension of an earlier method proposed for biochemical applications.

120 citations


Journal ArticleDOI
TL;DR: In this paper, the smallest number of placements of double bonds such that the full Kekule structure on the given parent graph is fully determined is considered, which is called the degree of freedom of the graph.
Abstract: For a Kekule structure we consider the smallest number of placements of double bonds such that the full Kekule structure on the given parent graph is fully determined. These numbers for each Kekule structure of the parent graph sum to a novel structural invariant F, called the degree of freedom of the graph. Some qualitative characteristics are identified, and it is noted that apparently it behaves differently from a couple of other invariants related to Kekule structures.

Journal ArticleDOI
Odd Gropen1
TL;DR: In this article, Gaussian basis sets of double zeta quality were presented for the fifth row elements, MoCd, and 19 s-type, 14 p-type (16 p-types), 10 d-type and 5 f-type functions for the sixth row elements.
Abstract: Gaussian basis sets, consisting of 17 s‐type, 12 p‐type, and 8 d‐type functions, for the fifth row elements, MoCd, and 19 s‐type, 14 p‐type (16 p‐type), 10 d‐type and 5 f‐type functions for the sixth row elements, WRn, are presented. The basis sets are of double zeta quality, and are optimized to .002 a.u. in the energy. The energies are compared with D.Z. STO basis sets.

Journal ArticleDOI
TL;DR: A survey of the progress in this area and some of the advantages and drawbacks of using topological indices can be found in this paper, where the authors discuss the manifold applications of topological invariants to the description of physicochemical properties.
Abstract: A class of graph invariants known today as topological indices are being increasingly realized by chemists and others to be powerful tools in the description of chemical phenomena. Topological indices generally characterize both the size and shape of chemical species; in recent years a number of such indices have been put forward that sensitively reflect the amount of branching present in molecules. Chemists are thus able to model accurately the chemical behavior of an extensive range of chemical substances in all three thermodynamic states. In discussing the manifold applications of topological indices to the description of physicochemical properties, we present a survey of the progress to date in this area and point out some of the advantages and drawbacks of using topological indices.

Journal ArticleDOI
TL;DR: In this paper, the build-up procedure for predicting low-energy conformations of polypeptides has been extended to cover the case of peptides in aqueous solutions.
Abstract: The build-up procedure for predicting low-energy conformations of polypeptides has been extended to cover the case of peptides in aqueous solutions. The revised procedure consists of five steps to be applied to each stage of the build-up. I. All low-energy minima of each of the two fragments to be joined are combined as starting points for energy minimization of the enlarged fragment, and those minima of the enlarged fragment within a certain upper bound of the lowest energy are retained. II. Whenever one of the combinations in Step I leads to an atomic overlap, the minimization is started again using a pseudoenergy function which remains finite everywhere and becomes equal to the standard energy function when no atoms overlap. III. The minima generated in Steps I and II are culled by ignoring side-chain conformations and retaining only those minima whose backbone conformations differ significantly. IV. The rotameric states of the side chains are optimized, by testing their energy of interaction with the rest of the molecule, and subjecting the whole molecule to a further round of energy minimization if the test indicates that this would reduce the energy. V. The energies of all minima are recomputed with inclusion of a term for solvation and with a smaller upper bound as the criterion for retention. The original build-up procedure consisted of Steps I and III only. Examples are presented showing the effectiveness of the new Steps II and IV in locating low-energy minima, and the problems that remain to be solved, chiefly concerning Step V, are discussed.

Journal ArticleDOI
TL;DR: In this paper, the spd-MPs were derived for the transition metal atoms Sc through Hg and the outermost core np electrons were treated explicitly together with valence nd and (n + 1)s electrons, and the remaining electrons were replaced by a model potential.
Abstract: Model potential parameters and valence orbitals were generated for the transition metal atoms Sc through Hg. They are named the spd-MPs and are supplementary to the sd-MPs presented in the preceding article. The outermost core np electrons were treated explicitly together with valence nd and (n + 1)s electrons, and the remaining electrons were replaced by a model potential. The model potential parameters and valence orbitals were determined in the same way as the sd-MPs. Major relativistic effects (via the mass velocity and Darwin terms) were also incorporated in the spd-MPs for the second-and third-row transition metal atoms. The results of numerical nonrelativistic Hartree-Fock (HF) calculations for the first-row transition metal atoms and of the quasirelativistic HF calculations with Cowan and Griffin's method for the second-row and third-row transition metal atoms were used as reference data in determination of the spd-MPs.

Journal ArticleDOI
TL;DR: In this paper, the MNDO Hamiltonian was used to predict dynamics for simple reactions, such as the path of steepest descent from the transition state backward to reactants and forward to products.
Abstract: The MNDO Hamiltonian as incorporated within MOPAC has been utilized to predict dynamics for some simple reactions. In one option, the intrinsic reaction coordinate has been followed along the path of steepest descent from the transition state backward to reactants and forward to products. In a second option, dynamics of isolated molecular systems have been calculated. In each case, the potential surface (as predicted by the MNDO Hamiltonian) is calculated in situ as the atomic trajectories are calculated from Newton's Laws of Motion. Several specific examples are given and discussed.

Journal ArticleDOI
TL;DR: The non-bonded aromatic portion of the MM3 force field has been derived to fit reasonably well the known properties for the above as mentioned in this paper, and the benzene dimer is calculated to have a lower energy when perpendicular, than when eclipsed face to face.
Abstract: Benzene has been studied with the MM3 force field, first as a monomer, then as a van der Waals dimer, then in the crystal. The non-bonded aromatic portion of the MM3 force field has been derived to fit reasonably well the known properties for the above. The force field includes dipoles of 0.6 and 0.9 D for CH and CC bonds, respectively. The benzene dimer is calculated to have a lower energy when perpendicular, than when eclipsed face to face. The crystal of benzene has a herringbone pattern with correct cell constants and heat of sublimation, while hexamethylbenzene as the dimer and in the crystal has a skewed-stack of pancakes type structure, as observed.

Journal ArticleDOI
TL;DR: In this paper, a new definition for a cooperativity parameter is proposed, based on the two-body, non-neighbour interaction energy, plus three- and four-body contributions, including one-body deformation terms in relation to the total interaction energy of the water tetramer.
Abstract: The additional energy stabilization due to cooperative effects was calculated in extended hydrogen bonded systems OH OH OH with unidirectional (homodromic) orientation of the OH groups. Ab initio restricted Hartree Fock, MP2 and MP3 calculations with geometry optimization and BSSE correction have been performed using the GAUSSIAN 83 program package for the ground states of the linear water dimer with Cs symmetry and the cyclic water tetramer with S4 symmetry. The latter represents the smallest possible, experimentally observed cooperative structure. A new definition for a cooperativity parameter is proposed. The definition is based on the two-body, non-neighbour interaction energy, plus three- and four-body contributions, including one-body deformation terms in relation to the total interaction energy of the water tetramer. The advantage of this definition is its independence of the reference system, which is necessary in complicated molecular systems with an undefined number of hydrogen bonds, such as disordered or flip-flop systems. According to this definition the energy gain based on cooperativity in the S4 water tetramer is 29% with the MP3/6-31G** approximation, (30% with HF/4-31G* and 46% with HF/3-21G). The largest contribution of 18% is due to the three-body term on the MP3/6-31G** level, followed by the two-body, non-neighbour term with 11%. The four-body term and the deformation term are in the order of 1% and cancel each other because they have opposite sign.

Journal ArticleDOI
TL;DR: In this article, the authors found that the 6.31G* and 6.311G* basis sets can shift the equilibrium intermolecular distance in H3CH•OH2 by up to 0.4 Å.
Abstract: Modifications of the standard 6‐31G** basis set as recommended in the accompanying paper are found to markedly lower the basis set superposition error (BSSE) in the title complexes, in contrast to enlargement to a triple‐ζ scheme or by addition of a diffuse sp shell or a second set of d‐functions without prior optimization, all of which lead to BSSE increase. After appropriate correction for correlation and superposition effects, all basis sets (with the exception of the standard 6‐31G** and 6‐311G** with their very large BSSE) predict the cyclic geometry of NH3 dimer to be more stable than the linear arrangement. Correlation and BSSE can shift the equilibrium intermolecular distance in H3CH‐OH2 by up to 0.4 Å. Failure to correct for superposition error leads to a drastic exaggeration of both the SCF and MP2 components of the interaction energy in this complex. Much better estimates are furnished by our recommended basis sets with their smaller superposition errors.

Journal ArticleDOI
TL;DR: In this paper, a good agreement between benzene dimer potentials calculated by MMP2 and ab initio SCF CI potentials was obtained using point charges of −0.15 on carbon and +0.5 on hydrogen.
Abstract: The MM2/MMP2 force‐field gives an incorrect description of benzene‐benzene interactions. This deficiency may be overcome by the addition of electrostatic interactions. Using point charges of −0.15 on carbon and +0.15 on hydrogen, a good agreement between benzene dimer potentials calculated by MMP2 and ab initio SCF CI potentials is obtained. The influence of the addition of point charges on calculated structures and conformational energies has been studied. Examples are given in which the addition of electrostatic interactions between phenyl groups substantially improves calculated conformational energies.

Journal ArticleDOI
TL;DR: A simple computer‐oriented method is presented for constructing the (molecular) distance matrix, where the distance matrix considered is the graph‐theoretical (topological)distance matrix.
Abstract: A simple computer-oriented method is presented for constructing the (molecular) distance matrix. The distance matrix considered is the graph-theoretical (topological) distance matrix.

Journal ArticleDOI
TL;DR: In this paper, the authors improved full ab initio optimizations of the molecular structure of biphenyl in twisted minimum energy, coplanar, and perpendicular conformations by use of Poles's GAUSSIAN 82 program.
Abstract: Improved full ab initio optimizations of the molecular structure of biphenyl in twisted minimum energy, coplanar, and perpendicular conformations by use of Poles's GAUSSIAN 82 program have been performed in the 6‐31G basis set. These lead to geometries and energies of much higher reliability than our earlier STO‐3G results. The torsional angle Φmin obtained now is 45.41° in close agreement with the recent experimental value of 44.4° ± 1.2°. Calculated CC distances may be converted to experimental ED rg‐values by means of independently determined linear regression correlations with very high statistical confidence, although they agree better with experimental x ray data for coplanar biphenyl without this correction. Calculated intramolecular angles are very similar for both STO‐3G and 6‐31G basis sets. The calculated torsional energy barrier towards Φ = 90° (ΔE90) is 6.76 kJ/mol in close agreement with the experimental‐31G value of 6.5 ± 2.0 kJ/mol. For coplanar biphenyl with D2h‐symmetry the calculated torsional energy barrier ΔE0 is 13.26 kJ/mol which is surprisingly much higher than the experimental value of 6.0 ± 2.1 kJ/mol. This discrepancy could not be resolved by optimizations assumed for two kinds of distortions of planarity of orthohydrogens from the molecular plane of the coplanar carbon atoms. But for the twisted minimum energy conformation asymmetric bending of ortho‐H atoms lead to a torsional angle Φmin = 44.74° together with a dihedral angle towards ortho‐H of 1.22°, and consequently even to an increase of torsional energy barriers to ΔE0 = 13.51 and ΔE90 = 6.91 kJ/mol.

Journal ArticleDOI
TL;DR: In this paper, simple modifications of basis sets of only moderate size (e.g., 6−31G) can accomplish the same end at much reduced computational expense, including reoptimization of the orbital exponents within the framework of the relevant molecules, plus addition of a single diffuse shell of sp orbitals on nonhydrogen centers.
Abstract: Basis set superposition error (BSSE) remains one of the major difficulties besetting current ab initio calculations of molecular interactions. Despite the widespread notion that lowering of the BSSE to negligible magnitude requires extremely large basis sets, we show that simple modifications of basis sets of only moderate size (e.g., 6‐31G**) can accomplish the same end at much reduced computational expense. These modifications include reoptimization of the orbital exponents within the framework of the relevant molecules, plus addition of a single diffuse shell of sp orbitals on nonhydrogen centers. Subsequent addition of a second set of d‐functions further lowers the SCF BSSE, bringing it below 0.1 kcal/mol for both (HF)2 and (H2O)2. It is notable that addition of the latter d‐functions without prior reoptimization of the valence orbitals produces the opposite effect of an increase in the BSSE. Although the MP2 BSSE is also substantially decreased by the above modifications, it appears difficult to reduce this quantity below about 0.4 kcal/mol.

Journal ArticleDOI
TL;DR: The ellipsoid algorithm was used in this article to refine the conformations of peptides in a constrained nonlinear optimization problem with NMR distance constraints, where the dihedral angles about single bonds were used as variables to keep the dimensionality low.
Abstract: A new method for constrained nonlinear optimization known as the ellipsoid algorithm is evaluated as a means of determining and refining the conformations of peptides. Advantages of the ellipsoid algorithm over conventional optimization methods include that it avoids many local minima that other methods would be trapped by, and that it is sometimes able to find optimum solutions in which the constraints are satisfied exactly. The dihedral angles about single bonds were used as variables to keep the dimensionality low (the rate of convergence decreases rapidly with increasing dimensionality of the problem). The method is evaluated on problems involving distance constraints, and for minimization of conformational energy functions. In an initial application, conformations consistent with an experimental set of NMR distance constraints were obtained in a problem involving 48 variable dihedral angles.

Journal ArticleDOI
TL;DR: In this article, a semi-empirical calculation for multiple hydrogen-bonded associations is presented. But, basic MNDO deficiencies and criteria applied for H-bonding result in some too high barriers for proton transfers.
Abstract: Recent suggestions for correcting HAcceptor interactions within MNDO, together with results of crystallographic analysis, were used to modify this SCF semiempirical calculation for multiple hydrogen bonded associations. Thermodynamic profiles for model systems of biochemical interest such as H2OH2O, hydration of neutral and charged molecules, dimerizations and proton transfers show the advantages of this method. Its treatment of charges, bonding, geometries, energetics and vibrational frequencies is shown to be comparable to ab initio calculations with various basis sets. However, basic MNDO deficiencies and criteria applied for H-bonding result in some too high barriers for proton transfers.

Journal ArticleDOI
Uzi Kaldor1
TL;DR: In this paper, the open-shell coupled-cluster method is applied to the electron affinities of Li and Na, which are calculated in two ways: as the ionization potential of the anions or as the energy of adding the second electron to the cations.
Abstract: The open-shell coupled-cluster method and the diagrams needed for its implementation are described. The method is applied to the electron affinities of Li and Na, which are calculated in two ways: as the ionization potential of the anions or as the energy of adding the second electron to the cations. The two schemes give essentially the same results, in very good agreement (<0.02 eV) with experiment. Three-body effects are negligible.

Journal ArticleDOI
TL;DR: In this article, the effect of polarization functions on disiloxane molecular orbital calculations at the 3.21G* level has been studied for ab initio calculations at 3.3G.
Abstract: The effect of polarization functions for ab initio molecular orbital calculations at the 3‐21G* level has been studied for disiloxane. Calculated molecular geometry, dipole moment, and the linearization barrier variation were analyzed for different uncontracted polarization functions. It was concluded that variation of the polarization function on oxygen has only a minor influence on the molecular properties of disiloxane, but its presence is required to obtain a bent geometry for the disiloxane bond. The calculated molecular properties of disiloxane are greatly influenced when the polarization function on silicon is varied. Two different values (0.3 and 0.9) for the exponent of the silicon polarization function provide results comparable to the experimental values for disiloxane. The only significant differences between the results obtained from ab initio calculations using the two polarization functions are in net atomic charges. The uncontracted polarization function of silicon with a value of 0.3 for its exponent is transferable to other organosilicon compounds. Calculated molecular geometries of flexible or rigid structures are in very good agreement with the experimental values.

Journal ArticleDOI
TL;DR: An improved algorithm is presented for rapid calculation of the hessian matrix for the conformational energy of a protein as a function of only dihedral angles, which is about one order faster than by the previous method.
Abstract: An improved algorithm is presented for rapid calculation of the hessian matrix for the conformational energy of a protein as a function of only dihedral angles. The speed of the calculation, which is about one order faster than by the previous method, is achieved by two considerations. First, the algorithm is designed to take advantage of the supercomputer pipeline architecture. Second, long-range, nonbonded interactions are cut off and long-range electrostatic interactions are approximated by dipole-dipole interactions in order to reduce the number of pairwise interactions that have to be computed. The results of benchmark tests of the program are given as applied for four globular proteins of different sizes.

Journal ArticleDOI
TL;DR: In this paper, it was found that the previous MM2 (or MMP2) description of out-of-plane deformation of aromatic rings sometimes yields distortions which are much too large.
Abstract: It has been found that the previous MM2 (or MMP2) description of out‐of‐plane deformation of aromatic rings sometimes yields distortions which are much too large. As a result, rotational barriers involving distortions of such rings may be calculated to have values which are too low. Examples are collected and discussed. An alternative formulation, which appears to significantly reduce the calculational error, is presented.

Journal ArticleDOI
TL;DR: Basis set expansion and correlation effects on computed hydrogen bond energies of the positive ion complexes AHn · AHn + 1+1, for AHn = NH3, OH2 and FH, have been evaluated in this article.
Abstract: Basis set expansion and correlation effects on computed hydrogen bond energies of the positive ion complexes AHn · AHn + 1+1, for AHn = NH3, OH2 and FH, have been evaluated. The addition of diffuse functions on nonhydrogen atoms is the single most important enhancement of split-valence plus polarization basis sets for computing hydrogen bond energies. Basis set enhancement effects appear to be additive in these systems. The correlation energy contribution to the stabilization energies of these complexes is significant, with the second order term being the largest term and having a stabilizing effect. The third order term is smaller and of opposite sign, while the fourth order term is smaller yet and stabilizing. As a result, computed MP4 stabilization energies are bracketed by the MP2 and MP3 energies. The overall effect of basis set enhancement is to decrease hydrogen bond energies, whereas the addition of electron correlation increases stabilization energies.