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Title of Journal: Theor Chem Acc

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Abbravation: Theoretical Chemistry Accounts

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Springer-Verlag

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10.1002/prac.19893310423

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1432-2234

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Can ORMAS be used for nonadiabatic coupling calcul

Authors: Aaron C West Theresa L Windus
Publish Date: 2012/07/24
Volume: 131, Issue: 8, Pages: 1251-
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Abstract

When appropriately used the multiconfigurational selfconsistent field MCSCF approximation is useful in discerning correct electronic structure results However with the increasing size of chemical systems of interest MCSCF rapidly becomes unfeasible due to the requirement of larger active spaces which lead to computationally unmanageable numbers of configurations This situation is especially true for complete active space selfconsistent field CASSCF In particular reducing this computational expense by using restricted active spaces in solving for gradients and nonadiabatic couplings NACs during dynamics runs would save computer time However the validity of such restricted spaces is not wellknown even for recovering the majority of the nondynamical correlation and inevitably varies between chemical systems across a range of nuclear geometries As such we use the recently implemented coupled perturbed–occupation restricted multiple active space CPORMAS equations West et al unpublished to verify the accuracy of this approximation for gradients and NACs vectors around two specific conical intersection geometries for the silaethylene and butadiene systems Overall no excitations between appropriate subspaces show qualitatively reasonable results while single excitations significantly improve ORMAS results relative to the CASSCF level in these particular systems However single excitation schemes do not always lead to the correct orbital subspaces and as a result seemingly smooth potential energy surfaces PES do not always result in smooth analytical gradients and NACs In addition while some of the single excitation ORMAS and CASSCF schemes have improper orbitals rotate into the active space the schemes without excitations even with more subspaces do not exhibit this behaviorThe authors are indebted to Michael W Schmidt and Mark S Gordon for help in using the capabilities of GAMESS and MCSCF This material is based upon work supported by the National Science Foundation under Grant No OISE0730114 for the partnerships in International Research and Education PIRE Theresa L Windus acknowledges computing resources purchased through funds provided by Ames Laboratory and Iowa State University

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