Lagrangian approach for geometrical derivatives and nonadiabatic coupling terms in MRCISD
Abstract
A new formulation for multireference configuration interaction method, with single and double excitations (MRCISD), energy gradients and nonadiabatic coupling terms based on a Lagrangian approach is suggested and the gradients demonstrated to be accurate with calculations on several molecules. Of particular interest, the suggested formalism can straightforwardly use stateaveragedmulticonfiguration selfconsistent field (SAMCSCF) descriptions of the reference space in which the states have arbitrary weights. This capability is demonstrated by some calculations on the ground and first excited ^{1}Σ^{+} states of LiH, including calculations near an avoided crossing. The formalism can use either complete or incomplete model (i.e. reference) spaces, and is limited, in this regard, only by the capabilities of the MCSCF program. The formalism for nonadiabatic coupling shares many structural features with the energy gradient formalism, with the use of a function that corresponds to the energy for the coupling.
 Publication:

Molecular Physics
 Pub Date:
 October 2010
 DOI:
 10.1080/00268976.2010.521777
 Bibcode:
 2010MolPh.108.2703K
 Keywords:

 electronic structure theory;
 energy gradients;
 multireference CI;
 nonadiabatic coupling