COUPLED-CLUSTER METHODS WITH NONITERATIVE TRIPLE EXCITATIONS FOR RESTRICTED OPEN-SHELL HARTREE-FOCK AND OTHER GENERAL SINGLE DETERMINANT REFERENCE FUNCTIONS - ENERGIES AND ANALYTICAL GRADIENTS

A new, noniterative triples correction to the coupled-cluster singles and doubles (CCSD), method, for general single determinant reference f unctions is proposed and investigated numerically for various cases, i ncluding non-Hartree-Fock (non-HF) reference functions. It is correct through fourth-order of perturbation theory for non-HF references, and unlike other such methods, retains the usual invariance properties co mmon to CC methods, while requiring only a single N7 step. In the cano nical Hartree-Fock case, the method is equivalent to the usual CCSD (T ) method, but now permits the use of restricted open-shell Hartree-Foc k (ROHF) and quasirestricted Hartree-Fock (QRHF) reference determinant s, along with many others. Comparisons with full configuration interac tion (FCl) results are presented for CH2, CH2+, CH3, NH2, and SiH2. Th e paper also reports the derivation and initial computational implemen tation of analytical gradients for the ROHF-CCSD(T) method, which incl udes unrestricted Hartree-Fock (UHF) CCSD(T) and RHF-CCSD(T) as specia l cases. Applications of analytical gradients are presented for HOO, t he CN radical, which is highly spin contaminated at the UHF level, and HCO, the latter with several large basis sets. With these development s of analytical gradients, these highly accurate generalized CCSD(T) m ethods can be widely applied.