Analytic first derivatives for general coupled-cluster and configuration interaction models

A general method for the analytic computation of energy gradients within coupled-cluster (CC) and configuration interaction (CI) theory is presented. The proposed algorithm is formulated in the framework of our recent string-based many-body formalism and allows to handle wave functions truncated at arbitrary excitation level or including excitations selected via a complete active space. The considered wave function classes include any single-reference as well as several multireference-type CC and CI ansatze and as a special case the full CI limit. The applicability of the proposed and implemented procedure is demonstrated by performing benchmark CC and CI gradient and property calculations up to full CI for H(2)O ((1)A(1)), HF ((1)Sigma(+)), NH(2) ((2)B(1)), and OH ((2)Pi) as well as for the transition state of the F+H(2)-->HF+H reaction. (C) 2003 American Institute of Physics.