RHF/ROHF

$closed shells

Specification of MO occupation for RHF, e.g.

 a1g     1-4                                    ( 2 )
 a2g     1                                      ( 2 )

$open shells type=1

MO occupation of open shells and number of open shells. 'type=1' here means that there is only a single open shell consisting e.g. of two MOs:

b2g     1                                      ( 1 )
b3g     1                                      ( 1 )

$rohf

This data group is necessary for ROHF calculations with more than one open shell. Example:

$rohf         1
    a -a    a=0  b=0
    h -h    a=1  b=2
    a -h    a=1  b=2

This example is for the 7S state of chromium ( 3d⁵ 4s¹) in symmetry group I. Note that for this option being activated, $roothaan also has to be specified in your control file, although its parameter has no meaning in this case. For more details see Section 6.3.

$roothaan

For ROHF-calculations with only one open shell the Roothaan parameters a and b have to be specified within this data group (see also $rohf). Example:

$roothaan
    a = 3/4      b = 3/2

This example is for the 3P ground state of carbon (2p²) in symmetry group I. define recognizes most cases and suggests good Roothaan parameters.

For further information on ROHF calculations (e.g. with more than one open shell), see the sample input in Section 16.6 and the tables of Roothaan parameters in Section 6.3.

Note that this keyword toggles the ROHF mode also for more than one open shell. If it is not given, the open-shell electrons are simply ignored.