8.9 Defining the wavefunction

In all program modules where such information is required, the total symmetry of the $N$-electron wavefunction is defined on WF (wavefunction) cards in the following way:

WF,nelec,irrep,spin

or, alternatively

WF,[NELEC=nelec],[SYM[METRY]=irrep],[spin=spin],[CHARGE=charge]

where nelec is the total number of electrons, irrep is the number of the irreducible representation, and spin equals $2 \times S$, where $S$ is the total spin quantum number. Instead of nelec also charge can be given, which specifies the total charge of the molecule. For instance, for a calculation in $C_{2v}$ symmetry with 10 electrons, WF,10,3,0 denotes a $^1B_2$ state, and WF,10,1,2 a $^3A_1$ state. The charge can also be defined by setting the variable CHARGE:

SET,CHARGE=charge

This charge will be used in all energy calculations following this input. Not that SET is required, since CHARGE is a system variable (cf. section 8.4).

Although in principle each program unit requires a WF command, in practice it is seldom necessary to give it. The program remembers the information on the WF card, and so one might typically specify the information in an SCF calculation, but then not in subsequent MCSCF or CI calculations; this also applies across restarts. Furthermore, nelec defaults to the sum of the nuclear charges, irrep to 1 and spin to 0 or 1; thus in many cases, it is not necessary to specify a WF card at all.