# [molpro-user] How to define occ, closed and core n1, n2, n3, n4, n5, n6, n7, n8?

Gerald Knizia knizia at theochem.uni-stuttgart.de
Tue May 31 14:56:01 BST 2011

```Wenjun Li wrote:
> One very key difficulty for me so far is that, I can never figure out
> how to define *occ, closed and core n1,n2,n3,n4,n5,n6,n7,n8.* For
> example for *occ,n1,n2,n3,n4,n5,n6,n7,n8*, ni is the number of
> occupied orbitals in the irreducible representation i. This is the
> only very short description for OCC in Molpro Manuals, but I can never
> figure out how to define *occ, closed and core n1,n2,n3,n4,n5,n6,n7,n8.*

What I tend to do is to run an rhf calculation via
{rhf; wf,sym=2,spin=1}
where sym is the irrep of the wave function (here 2) and spin is the
number of unpaired electrons (here 1). This will output its guess for
the number and irrep distributions of alpha (=occupied) and beta
(=doubly-occupied) orbtials at the end. In most cases it guesses right,
but sometimes you will need to re-occupy alpha or beta orbitals via the
closed/occ/open. You can also usually use these numbers to get started
if playing around with multi.
Note also that you can run a non-symmetry calculation (using
symmetry,nosym in the input) and see if you get the same result as with
the symmetry calculation.

Which physical symmetry operations the irreps correspond to is noted under
http://www.molpro.net/info/current/doc/manual/node33.html
For example, in Cs symmetry there is one mirror plane (z=0) as symmetry
operation. All functions invariant under reflection at z go into the A'
irrep (1) (e.g., s or p_x) and all functions changing sign during this
reflection (e.g., p_z, d_xz) go into A''.
--
Gerald Knizia

```