25 OPEN-SHELL COUPLED CLUSTER THEORIES

Spin unrestricted (RHF-UCCSD) and partially spin restricted (RHF-RCCSD) open-shell coupled cluster theories as described in J. Chem. Phys. 99 (1993) 5219 (see also erratum, J. Chem. Phys., submitted for publication) are available in MOLPRO. In both cases a high-spin RHF reference wavefunction is used. In the description that follows, the acronyms RCCSD and UCCSD are used, but the theories should normally be referred to as RHF-RCCSD, RHF-UCCSD, in order to distinguish them from alternative ansätze based on spin-unrestricted orbitals. The program will accept either the full or abbreviated acronyms as input commands.

In the RCCSD theory certain restrictions among the amplitudes are introduced, such that the linear part of the wavefunction becomes a spin eigenfunction (this is not the case in the UCCSD method, even if an RHF reference function is used). At present, the implementation of RCCSD is only preliminary, and no CPU time is saved by as compared to UCCSD. However, improved algorithms, as described in the above publication, are currently being implemented, and will be available in the near future.

The input is exactly the same as for closed-shell CCSD, except that RCCSD or UCCSD are used as keywords. By default, the open-shell orbitals are the same as used in the RHF reference function, but this can be modified using OCC, CLOSED, and WF cards.

Perturbative triples corrections are computed as follows:

RCCSD(T), UCCSD(T)

triples corrections are computed as defined by J. D. Watts, J. Gauss and R. J. Bartlett, J. Chem. Phys. 98 8718 (1993).

RCCSD[T], UCCSD[T]

corrections are computed without contributions of single excitations (sometimes called CCSD+T(CCSD)) .

RCCSD-T, UCCSD-T

triples corrections are computed as defined by M. J. O. Deegan and P. J. Knowles, Chem. Phys. Letters 227 (1994) 321.

In fact, all three contributions are always computed and printed. The following variables are used to store the results (here CCSD stands for either UCCSD or RCCSD):

ENERGY

total energy for method specified in the input.

ENERGC

total CCSD energy without triples.

ENERGT(1)

total CCSD(T) energy.

ENERGT(2)

total CCSD[T] energy.

ENERGT(3)

total CCSD-T energy.

We note that the present implementation performs the (partial) integral transformation using an algorithm which is not optimal in its memory use, and therefore needs quite a large memory if extensive basis sets are used. Future improvements will remove this bottleneck.