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Title: Molecular applications of analytical gradient approach for the improved virtual orbital-complete active space configuration interaction method
Authors: Chaudhuri, R. K
Chattopadhyay, S
Mahapatra, U. S
Freed, K. F
Keywords: Density Functional Theory;Gradient Methods;Ground States;HF Calculations;Orbital Calculations;Organic Compounds;Perturbation Theory;SCF Calculations;Total Energy;Triplet State;Vibrational States;Self-Consistent-Field Methods;Density-Functional Theory;Perturbation Theory;Potential Energy Surfaces;Excited Electronic States
Issue Date: Jan-2010
Publisher: American Institute of Physics
Citation: Journal of Chemical Physics, Vol. 132, No. 3, pp. 034105-1 - 034105-18
Abstract: The improved virtual orbital-complete active space configuration interaction (IVO-CASCI) method is extended to determine the geometry and vibrational frequencies for ground and excited electronic states using an analytical total energy gradient scheme involving both first and second order analytical derivatives. Illustrative applications consider the ground state geometries of the benzene (C6H6), biphenyl (C12H10), and alanine dipeptide (CH3CONHCHCH3CONHCH3) molecules. In addition, the IVO-CASCI geometry optimization has been performed for the first excited singlet (1B2u) and triplet states (3B1u) of benzene to assess its applicability for excited and open-shell systems. The D6h symmetry benzene triplet optimization produces a saddle point, and a descent along the unstable mode produces the stable minimum. Comparisons with Hartree-Fock, second order Möller-Plesset perturbation theory, complete active space self-consistent field (CASSCF), and density functional theory demonstrate that the IVO-CASCI approach generally fares comparable to or better for all systems studied. The vibrational frequencies of the benzene and biphenyl molecules computed with the analytical gradient based IVO-CASCI method agree with the experiment and with other accurate theoretical estimates. Satisfactory agreement between our results, other benchmark calculations, and available experiment demonstrates the efficacy and potential of the method. The close similarity between CASSCF and IVO-CASCI optimized geometries and the greater computational efficiency of the IVO-CASCI method suggests the replacement of CASSCF treatments by the IVO-CASCI approach, which is free from the convergence problems that often plague CASSCF treatments.
ISSN: 0021 - 9606
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