`TRIPLET`

The triplet state is defined. In order to define this type of calculation other keywords must also be used. For a 'simple' triplet calculation, use C.I.=2. Results from such calculations can be compared with ground state calculations. If the triplet state consists of two half-filled degenerate M.O.s, such as molecular oxygen, then OPEN(2,2) should be used.

From experience, OPEN(2,2) and TRIPLET run faster than C.I.=2 and TRIPLET.

If the system has an odd number of electrons, an error message will be printed.

See also MS=n, SINGLET, DOUBLET, QUARTET, QUINTET, SEXTET, SEPTET, OCTET, and NONET.

UHF interpretation:

The number of alpha electrons exceeds that of the beta electrons by 2. If TRIPLET is not specified, then the numbers of alpha and beta electrons are set equal. This does not necessarily correspond to a singlet.

RHF interpretation:

TRIPLET cannot be used unless other keywords are present. If C.I.=2 is used, then a single state corresponding to:

$begin{displaymath}frac{1}{sqrt{2}}(psi_{homo}^{alpha}.psi_{lumo}^{beta}+psi_{homo}^{beta}.psi_{lumo}^{alpha})end{displaymath}$

is calculated. See keywords C.I.=n and OPEN(n₁,n₂).

When the configuration interaction calculation is done, all microstates having a component of spin, M_S, equal to 1 are selected. These microstates are then used in the construction of states. Because of the way in which the microstates were chosen, only states of spin equal to or greater than 1 can be constructed. From this set, only a triplet state can be selected, all other states will be ignored. If ROOT=n is present, then the n'th triplet state will be selected, otherwise the first triplet state will be chosen.