Gas-Phase Molecular Structure of MBBA (4-Methoxybenzylidene-4‘-<i>n</i>-butylaniline), a Mesogen, by Electron Diffraction Combined with ab Initio Calculations
Journal of Physical Chemistry A, Apr 1, 1999
ABSTRACT
Molecular Structure of <i>p</i>-Azoxyanisole, a Mesogen, Determined by Gas-Phase Electron Diffraction Augmented by ab Initio Calculations
Journal of Physical Chemistry A, Feb 24, 1998
ABSTRACT
Vibrational spectra and conformation of cyclic methylsiloxanes
Spectrochimica acta, Mar 1, 1974
ABSTRACT Complete vibrational spectra (including far-i.r.) are given for the cyclic methyl-siloxa... more ABSTRACT Complete vibrational spectra (including far-i.r.) are given for the cyclic methyl-siloxanes from the trimer up to the hexamer. The assignment, given for hexamethylcyclotrisiloxane, is based on a planar structure, but some slight deviations from the strict D3h selection rules can be observed. The conformation of the higher members of the series is investigated in the light of flexibility of the rings.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, Jul 1, 1997
The method of Scaled Quantum Mechanical (SQM) force fields has been reviewed briefly and recent d... more The method of Scaled Quantum Mechanical (SQM) force fields has been reviewed briefly and recent developments to make its use more automatic discussed. Essential features are the definition of internal coordinates by a computer program and also a first version of assigning scale factors automatically. SQM calculations have been applied on three tautomers of the title molecule. Completely optimized geometries include results also with a large basis set, 6-31 lG(d, p). We find no convincing evidence for the presence of any other tautomer than the N,H imino form.
Practical aspects of the ab initio quantum chemical calculation of vibrational spectra
Journal of Molecular Structure-theochem, Apr 1, 1999
Scaled quantum mechanical (SQM) force field and vibrational assignment for styrene
Journal of Molecular Structure-theochem, Apr 1, 1994
ABSTRACT The geometry and complete harmonic force field for styrene were determined from ab initi... more ABSTRACT The geometry and complete harmonic force field for styrene were determined from ab initio Hartree-Fock calculations using the 4-21 gaussian basis set. Following the method of scaled quantum mechanical (SQM) force fields, systematic errors characteristic of this level of theory were corrected empirically. The scale factors were taken over fixed from benzene and butadiene so that the final SQM force field represents pure a priori results. Frequencies calculated from this force field reproduce the experimental values with a mean deviation of 13.9 and maximum deviations within 20cm−1. The torsional potential around the C-C bond between the ring and the vinyl group was also investigated, also using larger basis sets. All calculations give a gauche form as the minimum, but the potential is extremely shallowing thus justifying a vibrational treatment on the basis of planar geometry. For comparison, test calculations on cis- and gauche-butadiene are also reported.
ChemInform Abstract: EXTREMWERTE FUER KRAFTKONSTANTEN UND HAUPTSCHWINGUNGSAMPLITUDEN BEI MOLEKUELEN VOM AETHYLENTYP 2. MITT. TETRAFLUOR-, TETRACHLOR- UND TETRABROMAETHYLEN
The quadratic force field o f 2,6-dimethyl-1,5-dehydro-1,2,3-triazolo-[ 2 ,la ]-1,2,3triazole has... more The quadratic force field o f 2,6-dimethyl-1,5-dehydro-1,2,3-triazolo-[ 2 ,la ]-1,2,3triazole has been calculated from CNDO/2 wavefunctions by the force field method. Empirical scaling factors have been applied to obtain frequency fit. Using four different scaling factors, the average deviation between the calculated and experimental frequencies is 18.4 cm-1 for 41 assigned normal modes. The results demonstrate that the CNDO/2 force field method is suitable even for the normal coordinate analysis of relatively large molecules.
Please note that technical editing may introduce minor changes to the text and/or graphics, which... more Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal's standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
The UNO-CAS method, a computationally efficient approximation to CAS-SCF, has been applied to cal... more The UNO-CAS method, a computationally efficient approximation to CAS-SCF, has been applied to calculate completely optimized geometries for a variety of conjugated *-systems. The study includes all-trans-polyenes up to C12, molecules with heteroconjugation, with triple bonds and aromatics. Relative to SCF values, the bond lengths of multiple bonds are increased by 0.02-0.03 A, with a simultaneous smaller shortening of the single bonds (-0.005 A). The strong dependence of the geometry on electron correlation shows the importance of the latter for potential surface studies. Except for one case, glyoxal, the correlation is largely nondynamical rather than dynamical and can be well reproduced by UNO-CAS. Changes in the nature of conjugation in polyenes are close to convergence in dodecahexaene. The experimental geometries of some benzene derivatives and naphthalene are discussed.
An efficient direct method for geometry optimization of large molecules in internal coordinates
Journal of Chemical Physics, Oct 22, 1998
A new efficient procedure has been developed for geometry optimization of large molecules using i... more A new efficient procedure has been developed for geometry optimization of large molecules using internal coordinates. The method stores only the nonzero elements of the large transformation matrices, in the spirit of direct methods in electronic structure theory. Matrix inversion has been replaced by iterative solution of linear systems of equations by the preconditioned conjugate gradient method. A new incomplete Cholesky preconditioner proved essential to accelerate the conjugate gradient procedure. The geometries of several alpha helical alanine polypeptides, up to 50 alanine units, have been optimized by the new method, using the SYBYL force field. For larger systems, the number of energy/gradient evaluations is reduced by a factor of 6–10, compared to Cartesian optimization, and the cost of the optimization is small compared to the energy calculation. We expect this method to be useful in molecular mechanics and in mixed quantum mechanics/molecular mechanics calculations.
Fock matrix dynamics
Chemical Physics Letters, Mar 1, 2004
ABSTRACT An efficient method is suggested for direct ab initio Born–Oppenheimer molecular dynamic... more ABSTRACT An efficient method is suggested for direct ab initio Born–Oppenheimer molecular dynamics. It is based on extrapolating the Fock matrices from previous time steps to provide a start for the SCF procedure. Several polynomial extrapolation schemes are compared; low-degree schemes work best. SCF convergence can be achieved in 2–3, sometimes less than two cycles. We have identified a phenomenon, hysteresis, a violation of the energy conservation and reversibility, due to systematic errors arising from incomplete SCF convergence. It can be eliminated by rescaling the velocities. Examples include water clusters and hydrated halogen ions, running simulations for several picoseconds.
Journal of the American Chemical Society, Oct 1, 1992
Two suggestions are made to increase the efficiency and accuracy of ab initio optimization of mol... more 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.
Public reporting burden for this collection of information is estimated to average 1 hour per res... more Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Washington Headquarters Services, Directorate for Information Operations and Reports,
Scaled quantum mechanical (SQM) force field and vibrational assignment for cyclohexane
Spectrochimica acta, 1985
The complete harmonic force field of cyclohexane has been calculated ab initio, using the split v... more The complete harmonic force field of cyclohexane has been calculated ab initio, using the split valence 421 Gaussian basis set. To account for systematic errors, the ab initio force constants were scaled empirically. Altogether seven scale factors were used in this procedure, ...
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