## 5.1 Closed-shell Hartree-Fock calculations

The Hartree-Fock program is invoked by the command

hf

The program will then first compute the one-and two-electron integrals and store these on disk. Once this step is completed, the self-consistent field calculation, which is iterative, is carried out, using the precomputed integrals (for integral-direct calculations, in which the integrals are not stored but recomputed whenever needed, see section 15).

Now you are ready to try your first MOLPRO calculation. The complete input for a HF calculation for formaldehyde is

```***,formaldehyde
print,basis,orbitals   ! this is optional: print the basis set and
! the occupied orbitals
geometry={             ! define the nuclear coordinates
C
O  , C , rco
H1 , C , rch , O , hco
H2 , C , rch , O , hco , H1 , 180
}

rco=1.182 Ang
rch=1.102 Ang
hco=122.1789 Degree

basis=vdz              ! Select basis set
hf                     ! Invoke Hartree-Fock program
---
```

The first line, starting with *** is optional and is used to specify a title. The last line, -- is also optional. This indicates the end of the input; any further input after -- is ignored. Any text written after an exclamation mark is treated as comment and also ignored. Blanks and blank lines have no effect.

If you inspect the output, you will find that MOLPRO detected that the molecule has symmetry, and that there are 5 orbitals belonging to the irreducible representation , one belonging to , and two to . In MOLPRO the irreducible representations are numbered, and in the group the numbering is 1-4 for , , , and , respectively. The third orbital in symmetry is denoted 3.1, the second orbital in symmetry is denoted 2.3. Please take a little time to study the output in order to understand these conventions.

In the current case, MOLPRO has worked out the number of occupied orbitals in each symmetry automatically according to the Aufbau principle. This works in most but not all cases. You can specify the number of occupied orbitals in each symmetry using the occ directive. In the current case, it would read

occ,5,1,2

and the meaning of this should be quite obvious from the above.

If special directives are given for a command, like above occ is a directive for command hf, the command and the associated directives have to be enclosed by curly brackets, e.g.

```{hf                     ! Invoke Hartree-Fock program
occ,5,1,2}              ! Specify number of occupied orbitals in each irrep
```
This is called a command block. The curly brackets are needed to avoid ambiguities, since some directives can also be used outside command blocks. Please refer to the manual for more details.

Each command or directive can either begin on a new line, or be separated by semicolons. For instance

```{hf;occ,5,1,2}
```
is equivalent to the previous example.

molpro@molpro.net 2018-12-14