21.5 Level shifts

Level shifts are often useful to avoid intruder state problems in excited state calculations.
MOLPRO allows the use of shifts as described by Roos and Andersson,
[Chem. Phys. Lett. **245**, 215 (1995)].
The shift can be specified on the `RS2` or `RS2C` card

`RS2`[,`G`][,`SHIFT=`*shift*],`IPEA`=*value*
`RS2C`[,`G`][,`SHIFT=`*shift*],`IPEA`=*value*

Typical choices for the shift is are . Only two figures after the decimal point are considered.
The shift affects the results,
the printed energies as well as the `ENERGY` variable include the energy correction for the
shift as proposed by Roos and Andersson. At convergence, also the uncorrected energies
are printed for comparison.

Alternatively (or in addition), the IPEA shift of G. Ghigo, B. O. Roos, and
P.A. Malmqvist, Chem. Phys. Lett. **396**, 142 (2004) can be used. The implementation is not exactly
identical to the one in MOLCAS, since in our program the singly external configurations are not (RS2)
or only partially (RS2C) contracted. In Molpro, the shift is implemented as follows:

is added to the occupied part of the Fock matrix; in addition, is added as
a general shift (not corrected). is the value specified with the `IPEA` option (default 0).
A value of 0.20-0.25 is recommended. This removes intruder state problems to a large extent and usually
improves the results. Note that the method is not exactly orbital invariant, and pseudo-canonical orbitals
should be used (see `CANONICAL` option in MULTI).

It is possible to use `SHIFT` and `IPEA` simultaneously, but it does not make sense to use one
of the `G`-options together with `IPEA`.

molpro@molpro.net 2018-10-21