FORMULATION AND IMPLEMENTATION OF A RELATIVISTIC UNRESTRICTED COUPLED-CLUSTER METHOD INCLUDING NONITERATIVE CONNECTED TRIPLES

The formalism for a relativistic open-shell CCSD(T) method is presente d and implemented in a computer program, RELCCSD. The code can be used for calculations with 2- or 4-component relativistic reference wave f unctions and allows a full inclusion of the spin-orbit coupling. The c ode is interfaced to the MOLFDIR program system, We illustrate its use with nb initio calculations of the fine structure splittings of Cl, F O, ClO, O-2(+), and O-2(-). The triples correction is found to make a large contribution to the Cl atom splitting, which is within 23 cm(-1) , of the experimental value. The molecular results are within 4 cm(-1) of the experimental values where these are available, The value for F O is predicted to be -195+/-4 cm(-1), in good agreement with experimen t. (C) 1996 American Institute of Physics.