A Lagrangian, integral-density direct formulation and implementation of the analytic CCSD and CCSD(T) gradients

Using a Lagrangian formulation an integral-density direct implementation of the analytic CCSD(T) molecular gradient is presented, which circumvents the bottleneck of storing either O(N4) two-electron integrals or O(N4) density matrix elements on disk. Canonical orbitals are used to simplify the implementation of the frozen-core approximation and the CCSD gradient is obtained as a special case. Also a new, simplified approach to (geometrical) derivative integrals is presented. As a first application we report a full geometry optimization for the most stable isomer of SiC3 using the cc-pV5Z basis set with 368 contracted basis functions and the frozen-core approximation.