Gauge invariant coupled cluster response theory using optimized nonorthogonal orbitals

Using the time-dependent Lagrangian response approach, the recently revived orbital optimized coupled cluster (OCC) model is reformulated using nonort hogonal orbital rotations in a manner that conserves the commutativity of t he cluster excitation operators. The gauge invariance and the simple pole s tructure of the OCC linear response function are retained, while the dimens ion of the eigenvalue problem is reduced by a factor of 2. Restricting the cluster operator to double excitations, we have carried out the first imple mentation of gauge invariant coupled cluster response theory. Test calculat ions of the excitation energy, and length and velocity gauge oscillator str engths are presented for the lowest electric dipole allowed transitions of the CH+ molecular ion and the Ne atom. Additionally, the excitation energie s to the four lowest-lying states of water are calculated. (C) 2001 America n Institute of Physics.