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19.4.5 Projection to specific $\Lambda$ states in linear molecules

Since MOLPRO can only use Abelian point groups (e.g. $C_{2v}$ instead of $C_{\infty v}$ for linear molecules), $\Delta_{x^2-y^2}$ states as well as $\Sigma^+$ states occur in the irreducible representation number 1, for example. Sometimes it is not possible to predict in advance to which state(s) the program will converge. In such cases the LQUANT option can be used to specify which states are desired.

LQUANT,lam(1),lam(2),$\ldots$,lam(nstate);

lam(i) is the $\Lambda$ quantum number of state $i$, i.e., 0 for $\Sigma$ states, 1 for $\Pi$ states, 2 for $\Delta$ states, etc. The matrix over $\Lambda^2$ will be constructed and diagonalized in the P-space configuration basis. The eigenvectors are used to transform the P-space hamiltonian into a symmetry adapted basis, and the program then selects the eigenvectors of the correct symmetry. The states will be ordered by symmetry as specified on the LQUANT card; within each symmetry, the states will be ordered according to increasing energy.

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