MP2 natural orbitals module by S. Knecht and H. J. Aa. Jensen.

The MP2 calculation will produce the MP2 energy and the natural orbitals for the
density matrix through second order. The primary purpose of this option is to generate
good starting orbitals for CI or MCSCF but also CC wave functions, but it may of course also be used to
obtain the MP2 energy, perhaps with frozen core orbitals. For valence correlation calculations
it is recommended that the core orbitals are excluded via the *.ACTIVE* in order to obtain the appropriate
correlating orbitals as start for an MCSCF calculation. As the commonly used basis sets
do not contain correlating orbitals for the core orbitals and as the core correlation energy
therefore becomes arbitrary, a thoughtfully chosen *.ACTIVE* option can also be of benefit in MP2 energy
calculations. See also Ref. [Jensen1988] for more details.

*Note*: the module works at present only for closed-shell Hartree-Fock reference wave functions.

Read *Specification of orbital strings* of
orbitals to be included in the MP2 calculation when constructing
the MP2 density and natural orbitals.

Maximum number of virtual orbitals in the MP2 calculation. The actual number will be reduced to this value
if its exceeded by the number calculated from *.ACTIVE*.

*Default:*

```
.MAX VS
200
```

Select a minimal set of natural orbitals given as a string analog to the *Specification of orbital strings*
defined as active orbitals in subsequent correlation calculations. The numbers are: max. occupation, min occupation,
safety tolerance (here +-10%).

*Default*:

```
.SEL NO
NO-occ 1.99 0.001 0.1
```

Sets the integral transformation scheme for the ***MOLTRA* part. *Default*: default scheme of MOLTRA (scheme 6):

```
.SCHEME
6
```

Perform a Mulliken population analysis (see **MULPOP*) of the MP2 natural orbitals. This can be useful for a
comparison with a Mulliken population analysis of the Hartree-Fock orbitals.

Specify what two-electron integrals to include during the construction
of the MP2 natural orbitals (default: *.INTFLG* under ***HAMILTONIAN*).