MOLPRO

16 THE SCF PROGRAM 105
16.6 Using additional point-group symmetry
Since MOLPRO can handle only Abelian point-groups, there may be more symmetry than explicitly
used. For instance, if linear molecules are treated in C 2v instead of C ∞v , the δ (x 2 −y 2 )-
orbitals appear in symmetry 1 (A 1 ). In other cases, a linear geometry may occur as a special
case of calculations in C S symmetry, and then one component of the π-orbitals occurs in symmetry
1 (A ′ ). The program is able to detect such hidden “extra” symmetries by blockings in the
one-electron hamiltonian h and the overlap matrix S. Within each irreducible representation, an
“extra” symmetry number is then assigned to each basis function. These numbers are printed
at the end of the integral output. Usually, the extra symmetries are ordered with increasing l-
quantum number of the basis functions. This information can be used to determine and fix the
extra symmetries of the molecular orbitals by means of the SYM command.
SYM,irrep,sym(1),sym(2),,,sym(n)
sym(i) are the extra symmetries for the first n orbitals in the irreducible representation irrep. For
instance, if you want that in a linear molecule the orbitals 1.1 to 3.1 are σ and 4.1, 5.1 δ, the
SYM card would read (calculation done with X,Y as symmetry generators):
SYM,1,1,1,1,2,2
If necessary, the program will reorder the orbitals in each iteration to force this occupation. The
symmetries of occupied and virtual orbitals may be specified. By default, symmetry contaminations
are not removed. If irrep is set negative, however, symmetry contaminations are removed.
Note that this may prevent convergence if degenerate orbitals are present.
16.7 Expectation values
EXPEC,oper 1 ,oper 2 ,...,oper n
Calculates expectation values for one-electron operators oper 1 , oper 2 , ..., oper n . See section
6.13 for the available operators. By default, the dipole moments are computed. Normally, it
is recommended to use the GEXPEC directive if expectation values for other operators are of
interest. See section 6.13 for details.
16.8 Polarizabilities
POLARIZABILITY[,oper 1 ,oper 2 ,...,oper n ]
Calculates polarizabilities for the given operators oper 1 , oper 2 , ..., oper n .. See section 6.13 for
the available operators. If no operators are specified, the dipole polarizabilities are computed.
Presently, this is working only for closed-shell without direct option.
The polarizabilities are stored in the variables POLXX, POLXY, POLXZ, POLYY, POLYZ,
POLZZ.
16.9 Miscellaneous directives
All commands described in this section are optional. Appropriate default values are normally
used.