A description of the ab initio quantum chemistry package GAMESS is presented. Chemical systems containing atoms through radon can be treated with wave functions ranging from the simplest closed-shell case up to a general MCSCF case, permitting calculations at the necessary level of sophistication. Emphasis is given to novel features of the program. The parallelization strategy used in the RHF, ROHF, UHF, and GVB sections of the program is described, and detailed speecup results are given. Parallel calculations can be run on ordinary workstations as well as dedicated parallel machines. © John Wiley & Sons, Inc.

General atomic and molecular electronic structure system

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.

Using redundant internal coordinates to optimize equilibrium geometries and transition states

A linear synchronous transit or quadratic synchronous transit approach is used to get closer to the quadratic region of the transition state and then quasi-newton or eigenvector following methods are used to complete the optimization. With an empirical estimate of the hessian, these methods converge efficiently for a variety of transition states from a range of starting structures.

Combining Synchronous Transit and Quasi-Newton Methods to Find Transition States

The electronic Ligand Builder and Optimization Workbench (eLBOW) is a program module of the PHENIX suite of computational crystallographic software. It is designed to be a flexible procedure that uses simple and fast quantum-chemical techniques to provide chemically accurate information for novel and known ligands alike. A variety of input formats and options allow the attainment of a number of diverse goals including geometry optimization and generation of restraints.

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electronic Ligand Builder and Optimization Workbench (eLBOW): a tool for ligand coordinate and restraint generation

We present an alternative approach to the derivation of scaled quantum mechanical (SQM) force fields involving the direct scaling of individual primitive valence force constants from a full set of redundant valence coordinates. Our approach is completely general and more flexible than previous SQM schemes. Optimal scaling factors for various primitive stretching, bending, and torsional force constants are derived from a training set of 30 molecules containing C, O, N, H, and Cl and used to scale force constants for a further 30 molecules. Calculated vibrational frequencies are compared with experimental values for over 1500 fundamentals. Using the hybrid three-parameter B3-LYP density functional with the split-valence 6-31G* basis set, our scaling procedure gives an average error of less than 8.5 cm-1 in the scaled frequencies. The average percentage error is under 1%.

Direct Scaling of Primitive Valence Force Constants: An Alternative Approach to Scaled Quantum Mechanical Force Fields

Two suggestions are made to increase the efficiency and accuracy of ab initio optimization of molecular geometries. To improve the convergence of the optimization, a set of internal coordinates, the natural valence coordinates, is suggested. These coordinates originate from vibrational spectroscopy and reduce both harmonic and anharmonic coupling terms in the potential function as much as possible in a purely geometrical definition. The natural valence coordinates are local, eliminate most redundancies, and conform to local pseudosymmetry. Special attention has been paid to ring systems. A computer program has been included in our program system TX90 to generate the natural internal coordinates automatically. The usefulness of these coordinates is demonstrated by numerous examples of ab initio geometry optimization. Starting with a geometry preoptimized by molecular mechanics and using a simple diagonal estimate of the Hessian in conjunction with the GDIIS optimization technique, we usually achieved convergence in 8-15 steps, even for large molecules. It is demonstrated that, due to the reduction in anharmonic couplings, natural coordinates are superior to Cartesian or other simple internal coordinates, even when an accurate initial Hessian is available. Constrained optimization and the location of transition states are also discussed. The gradient optimization method has been generalized to handle redundancies; this is necessary in some complex polycyclic molecules and is illustrated on, among others, the porphine molecule. To increase the accuracy of relatively low-level calculations, empirical corrections to ab initio SCF geometries are suggested in the form of “offset forces” acting along bonds. We recommend offset forces for the most important bonds, to be used with the 4-21G(*) and the 6-31G* basis sets. Based on 130 comparisons, the mean absolute error between theoretical and experimental bond lengths is reduced this way from 0.014 to 0.005 A.

The calculation of ab initio molecular geometries: efficient optimization by natural internal coordinates and empirical correction by offset forces

The geometry and harmonic force field of the phenoxyl radical are determined by ab initio calculations of the electronic structure. All the π electrons are involved in strong nondynamical correlations, which are treated in a complete π active space wave function. Results from using multiconfigurational self‐consistent‐field orbitals in the active space are found to be closely approximated by use of the more economical unrestricted Hartree–Fock natural orbitals. Large polarized basis sets are also required to obtain qualitatively correct results. Properties of the carbonyl chromophore are particularly interesting. Compared to closed‐shell analogs, the equilibrium geometry indicates the CO bond in phenoxyl to be nearly as short as a typical double bond, whereas the scaled quantum‐mechanical force field shows it to have strength intermediate between those of typical single and double bonds. The calculated fundamental vibrational frequencies generally confirm current interpretations of the major features of the observed experimental resonance Raman spectra, while leading to reassignments of some minor features. Some limited theoretical characterization of valence excited states provides qualitative explanation of the observed intensity pattern. Calculations carried out at lower levels of theory consistently find the CO bond to be too long and weak, leading to misassignment of the observed vibrational spectrum.

Structure and fundamental vibrations of phenoxyl radical

Ab initio prediction of vibrational spectra: A database approach

Ab initio prediction of vibrational spectra: a database approach

The recently developed unrestricted Hartree-Fock natural orbital-complete active space (UNO-CAS) method is applied to the determination of the geometry and force field of p-benzoquinone. With an active space consisting of eight electrons distributed in eight π orbitals (8×8) and the 6-31G* basis set, UNO-CAS predicts CC and CO distances in good agreement with gas electron diffraction results. The full quadratic force field is obtained by an empirical scaling of the 8×8 UNo-CAS/4-21G force constants evaluated at the 6-31G* UNO-CAS geometry

Xuefeng Zhou

Papers

The calculation of ab initio molecular geometries: efficient optimization by natural internal coordinates and empirical correction by offset forces

Structure and fundamental vibrations of phenoxyl radical

Ab initio prediction of vibrational spectra: A database approach

Ab initio prediction of vibrational spectra: a database approach

Assignment of the fundamental frequencies of p-benzoquinone : a scaled quantum mechanical force field study