In the CC formulation, there is no coupling between channels whose internal wavefunction have opposite total parity. JLPAR is the parameter which selects the parity of the channels which are included in a CC calculation.

If JLPAR=+1 or -1, then for molecules in electronic states of odd multiplicity (integer spin) only those channels for which

e(-1) J - S - s + L - Jtot = JLPAR

are included in the channel basis. For molecules in electronic states of even multiplicity (half-integer spin) only those channels for which

e(-1)J - S - s + L - Jtot - 1/2 = JLPAR

are included in the channel basis. Here

• e = symmetry index of molecular state in Hund’s case (a) (e = +1 or -1)
• S = total spin
• s = 1 for sigma-minus states, 0 otherwise
• L = orbital angular momentum

❗ In general, in the CC formulation, calculations for both values of JLPAR must be performed to determine integral and/or differential cross sections. Setting JLPAR = 0 in a CC calculation will ensure that the calculation is done first for JLPAR = 1, and subsequently for JLPAR = -1.

❗ The parameter JLPAR is ignored in coupled states calculations.