Analytic gradients for Mukherjee's multireference coupled-cluster method using two-configurational self-consistent-field orbitals.

Analytic gradients for the state-specific multireference coupled-cluster method suggested by Mahapatra et al. [Mol. Phys. 94, 157 (1998)] (Mk-MRCC) are reported within the singles and doubles approximation using two-configurational self-consistent field (TCSCF) orbitals. The present implementation extends our previous work on Mk-MRCC gradients [E. Prochnow et al., J. Chem. Phys. 131, 064109 (2009)] which is based on restricted Hartree-Fock orbitals and consequently the main focus of the present paper is on the treatment of orbital relaxation at the TCSCF level using coupled-perturbed TCSCF theory. Geometry optimizations on m-arynes and nitrenes are presented to illustrate the influence of the orbitals on the computed equilibrium structures. The results are compared to those obtained at the single-reference coupled-cluster singles and doubles and at the Mk-MRCC singles and doubles level of theory when using restricted Hartree-Fock orbitals.