APPLICATION OF QUADRATIC-CI WITH SINGLES, DOUBLES, AND TRIPLES (QCISDT) - AN ATTRACTIVE ALTERNATIVE TO CCSDT

The size-extensive quadratic cr method with single (s), double (D), an d triple (T) excitations, QCISDT, is compared with QCISD, QCISD(T), CC SDT-IZ, and CCSDT. It is shown that QCISDT results are more accurate t han are either QCISD or QCISD(T) results. In particular, QCISDT turns out to be more stable than are QCISD and QCISD(T) in cases with consid erable multireference character. QCISDT and CCSDT results are of simil ar accuracy with slight advantages for the former method. Since QCISDT is much easier to implement on a computer than is CCSDT, it is an att ractive alternative to CCSDT. (C) 1996 John Wiley & Sons, Inc.