# How to specify complete active spaces in the Dalton software package? [closed]

What do the numbers for specifying the inactive and active space of a Dalton calculation mean?

For example, I do not understand what the following numbers refer to:

.INACTIVE Doubly occupied orbitals
1 0 0 0 1s on oxygen

.CAS SPACE
4 2 2 0

• Dalton manual p.14 has explanations for these values. – Bdrs Apr 18 '18 at 15:30

I am guessing that this is an example taken from the Dalton manual on page 13. The complete listing is

**DALTON INPUT
.OPTIMIZE
**WAVE FUNCTIONS
.HF
.MP2
.MCSCF
*SCF INPUT
.DOUBLY OCCUPIED
3 1 1 0
*CONFIGURATION INPUT
.SYMMETRY
1
.SPIN MULTIPLICITY
1
.INACTIVE
1 0 0 0
.CAS SPACE
4 2 2 0
.ELECTRONS
8
**START
.SHIELD
.MAGNET
**PROPERTIES
.DIPGRA
.VIBANA
**END OF DALTON INPUT


There is a lot of unimportant stuff for your question, but we do a calculation on a water molecule, represented in the C2v point group, so we have 4 irreps.
We want to keep the 1s electrons on the oxygen doubly occupied, and to do this, we write

.INACTIVE
1 0 0 0


This means that there is one inactive (doubly occupied) orbital in the A1 irrep, while none of the B1, B2 or A2 irreps have any irreps forced to be doubly occupied.

This corresponds to the 1s orbital on the oxygen atom, which is so much lower in energy that is acceptable to not correlate.

A water molecule has 10 electrons, and since we have already used two to fill the doubly occupied 1s orbital on oxygen, we have 8 electrons left. These are distributed according to the specification

.CAS SPACE
4 2 2 0


This means that we have 4 orbitals with A1 symmetry, 2 with B1, 2 with B2 and 0 with A2 symmetry, which we would like to correlate.