Contracted Gaussian basis sets of triple zeta valence (TZV) quality are presented for Li to Kr. The TZV bases are characterized by typically including a single contraction to describe inner shells and three basis functions for valence shells. All parameters—orbital exponents and contraction coefficients—have been determined by minimization of atomic self‐consistent field ground state energies. Advantages and necessary modifications of TZV basis sets are discussed for simple test calculations of molecular energies and nuclear magnetic resonance (NMR) chemical shieldings in treatments with and without inclusion of electron correlation.

Fully optimized contracted Gaussian basis sets of triple zeta valence quality for atoms Li to Kr

Scaling factors for obtaining fundamental vibrational frequencies, low-frequency vibrations, zero-point vibrational energies (ZPVE), and thermal contributions to enthalpy and entropy from harmonic frequencies determined at 19 levels of theory have been derived through a least-squares approach. Semiempirical methods (AM1 and PM3), conventional uncorrelated and correlated ab initio molecular orbital procedures [Hartree−Fock (HF), Moller−Plesset (MP2), and quadratic configuration interaction including single and double substitutions (QCISD)], and several variants of density functional theory (DFT:  B-LYP, B-P86, B3-LYP, B3-P86, and B3-PW91) have been examined in conjunction with the 3-21G, 6-31G(d), 6-31+G(d), 6-31G(d,p), 6-311G(d,p), and 6-311G(df,p) basis sets. The scaling factors for the theoretical harmonic vibrational frequencies were determined by a comparison with the corresponding experimental fundamentals utilizing a total of 1066 individual vibrations. Scaling factors suitable for low-frequency vib...

Harmonic Vibrational Frequencies: An Evaluation of Hartree−Fock, Møller−Plesset, Quadratic Configuration Interaction, Density Functional Theory, and Semiempirical Scale Factors

In this study, we present conformational energies for a molecular mechanical model (Parm99) developed for organic and biological molecules using the restrained electrostatic potential (RESP) approach to derive the partial charges. This approach uses the simple "generic" force field model (Parm94), and attempts to add a minimal number of extra Fourier components to the torsional energies, but doing so only when there is a physical justification. The results are quite encouraging, not only for the 34-molecule set that has been studied by both the highest level ab initio model (GVB/LMP2) and experiment, but also for the 55-molecule set for which high-quality experimental data are available. Considering the 55 molecules studied by all the force field models for which there are experimental data, the average absolute errors (AAEs) are 0.28 (this model), 0.52 (MM3), 0.57 (CHARMm (MSI)), and 0.43 kcal/mol (MMFF). For the 34-molecule set, the AAEs of this model versus experiment and ab initio are 0.28 and 0.27 kcal/mol, respectively. This is a lower error than found with MM3 and CHARMm, and is comparable to that found with MMFF (0.31 and 0.22 kcal/mol). We also present two examples of how well the torsional parameters are transferred from the training set to the test set. The absolute errors of molecules in the test set are only slightly larger than in the training set (differences of <0.1 kcal/mol). Therefore, it can be concluded that a simple "generic" force field with a limited number of specific torsional parameters can describe intra- and intermolecular interactions, although all comparison molecules were selected from our 82-compound training set. We also show how this effective two-body

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How Well Does a Restrained Electrostatic Potential (RESP) Model Perform in Calculating Conformational Energies of Organic and Biological Molecules

A potential function is presented that can be used to model both chemical reactions and intermolecular interactions in condensed-phase hydrocarbon systems such as liquids, graphite, and polymers. This potential is derived from a well-known dissociable hydrocarbon force field, the reactive empirical bond-order potential. The extensions include an adaptive treatment of the nonbonded and dihedral-angle interactions, which still allows for covalent bonding interactions. Torsional potentials are introduced via a novel interaction potential that does not require a fixed hybridization state. The resulting model is intended as a first step towards a transferable, empirical potential capable of simulating chemical reactions in a variety of environments. The current implementation has been validated against structural and energetic properties of both gaseous and liquid hydrocarbons, and is expected to prove useful in simulations of hydrocarbon liquids, thin films, and other saturated hydrocarbon systems.

https://www.usna.edu/Users/chemistry/jah/_files/Papers/AIREBO_JCPversion.pdf

A reactive potential for hydrocarbons with intermolecular interactions

An MCSCF procedure is described which is based on the direct minimization of an approximate energy expression which is periodic and correct to second order in the changes in the orthonormal orbitals Within this approximation, the CI coefficients are fully optimized, thereby accounting for the coupling between orbital rotations and CI coefficients to higher order than in previous treatments Additional transformations among the internal orbitals and their associated one‐ and two‐electron integrals are performed which amounts to treating the rotations among internal orbitals to higher than second order These extra steps are cheap compared to the four index transformation performed in each iteration, but lead to a remarkable enhancement of convergence and overall efficiency In all calculations attempted to date, convergence has been achieved in at most three iterations The energy has been observed to converge better than quadratically from the first iteration even when the initial Hessian matrix has many negative eigenvalues

A second order multiconfiguration SCF procedure with optimum convergence

The improved canonical variational theory, presented previously for reaction rates in a classical mechanical world, has been extended to reactions in a quantum mechanical world where internal energies of reactants are quantized. A detailed discussion of vibrationally adiabatic models for transmission coefficients for conventional transition-state theory and three versions of variational transition-state theory are also presented. The relationship of the improved canonical variational transition-state theory to the classical limit of these transmission coefficients has been shown. The improved canonical variational theory and the new quantal and classical transmission coefficients are illustrated and tested by applications to quantal collinear and three-dimensional reactions rates for several reactions. Semiclassical approximations to the quantal transmission coefficients are also examined. Applications considered are collinear H + H/sub 2/ and isotopic analogs, Cl + H/sub 2/ and isotopic analogs, and I + H/sub 2/ and three-dimensional D + H/sub 2/, Cl + HD, I + H/sub 2/, I + D/sub 2/, O + H/sub 2/, and F + H/sub 2/.

Improved treatment of threshold contributions in variational transition-state theory

For comparison with experimentally obtained thermochemical data, zero‐point vibrational energies (ZPVEs) are required to convert total electronic energies obtained from ab initio quantum mechanical studies into 0 K enthalpies. The currently accepted practice is to employ self‐consistent‐field (SCF) harmonic frequencies that have been scaled to reproduce experimentally observed fundamental frequencies. This procedure introduces systematic errors that result from a recognizable flaw in the method, namely that the correct ZPVE, G(0), is not one half the sum of the fundamental vibrational frequencies. Until better methods for accurately determining ZPVEs are presented, we recommend using different scaling factors for the determination of ZPVEs than those used to compare theoretically determined harmonic frequencies to observed fundamentals.

Concerning zero‐point vibrational energy corrections to electronic energies

We have discovered a low‐lying cyclic isomer of Si–C–C which is best described as a three‐membered ring with a weak carbon–carbon triple bond. In these theoretical studies a double zeta plus polarization basis set was used initially. At the self‐consistent‐field (SCF) and two‐configuration (TC) SCF levels of theory the ring structure is a transition state leading to linear Si–C–C. A configuration interaction (CI) treatment at the SCF optimized cyclic geometry, however, show it to lie 1.1 kcal/mol below the linear structure. Extension of the basis set to include a second set of d functions on each atom gives a final prediction that the ring structure lies ∼5 kcal below linear SiCC.

An energetically low‐lying silacyclopropyne isomer of SiC2

Inspired by the observation of a monobridged structure of Si2H2 by Cordonnier et al. via microwave spectroscopy (see the following paper), we have reinvestigated the Si2H2 singlet state potential energy surface using large basis sets and extensively correlated wave functions. Coupled‐cluster single, double, and (perturbative) triple excitation methods [CCSD(T)] in conjunction with a triple‐zeta 2df (TZ2df ) basis set on silicon and a triple zeta with two sets of polarization (TZ2P) basis set on hydrogen predict that the monobridged Si(H)SiH structure is indeed a minimum; in fact, Si(H)SiH is the second most stable Si2H2 isomer, as suggested by a recent theoretical study [B. T. Colegrove and H. F. Schaefer, J. Phys. Chem. 94, 5593 (1990)]. The predicted Si(H)SiH geometrical structure—which exhibits the shortest SiSi bond distance of any molecule characterized to date—and hence the rotational constants, as well as the quartic centrifugal distortion constants are in good agreement with the experimental data. We have located transition states between these pairs of minima—disilavinylidene H2SiSi and monobridged Si(H)SiH; monobridged and dibridged Si(H2)Si; trans‐HSiSiH and monobridged. We predict Si(H)SiH to lie 8.7 kcal mol−1 above Si(H2)Si, with the transition state between them 3.7 kcal mol−1 higher. H2SiSi is predicted to lie 11.6 kcal mol−1 above Si(H2)Si and the transition state barrier between H2SiSi and Si(H)SiH is 2.4 kcal mol−1 above H2SiSi. Predictions of absolute 0 K heats of formation for the various structures are presented.

The remarkable monobridged structure of Si2H2

To simulate and help interpret the nature of the newly synthesized Ga2R2Na2 molecule with bulky groups, ab initio and density functional quantum mechanical methods were applied to study the structures and bonding of the model [HGaGaH]2-, [H2GaGaH2]2-, and [H3CGaGaCH3]2- dianions, as well as the neutral Na2[H2GaGaH2], Na2[H3CGaGaCH3], Ga2H2, and Ga2H4 species. Basis sets of triple-ζ plus double polarization quality augmented with diffuse functions were employed. No general bond lengthbond order relationship is found. Bending from linearity of the acetylene analogues increases the GaGa separation more than the bond order is decreased. The GaGa bonding in the experimental molecule is concluded to be between triple and double in character despite the relatively long bond length.

Roger S. Grev

Papers

Improved treatment of threshold contributions in variational transition-state theory

Concerning zero‐point vibrational energy corrections to electronic energies

An energetically low‐lying silacyclopropyne isomer of SiC2

The remarkable monobridged structure of Si2H2

The Nature of the Gallium−Gallium Triple Bond