Description of Commands

s list

This command gives you a listing of all MOs and their energies as obtained from the extended Hückel calculation. For NH₃ in C_3v and TZVP you get, e.g.:

ORBITAL SYMMETRY  ENERGY      SHELL   CUMULATED  CL.SHL OCC.  OP.SHL OCC.
(SHELL)   TYPE             DEGENERACY SHELL DEG. PER ORBITAL  PER ORBITAL
    1    1a1      -15.63244      2           2      0.0000      0.0000
    2    2a1       -0.99808      2           4      0.0000      0.0000
    3    1e        -0.64406      4           8      0.0000      0.0000
    4    3a1       -0.57085      2          10      0.0000      0.0000
    5    2e         0.30375      4          14      0.0000      0.0000
    6    4a1        0.87046      2          16      0.0000      0.0000
TO CONTINUE, ENTER <return>

p index

This allows you to get the linear combination of basis functions which form the MO-index. Note that this refers not to the basis set you specified, but to the extended Hückel basis. index must be a single index, not an index list.

c list

This command allows you to specify closed shells. Their occupation will be 2 per MO, the total occupation the shell degeneracy which you can obtain by using command s. list is a list of shell indices like 1-13 or 1,3-5,7.

o index

This command allows you to specify open shells. index must be a single shell index, not an index list. You will then be asked for the number of electrons per MO which shall be contained in this shell. For example, for a fluorine atom you should choose o n (where n is the index of the p-shell) and an occupation of 5/3 per MO. You may enter the occupation numbers as simple integers or as integer fractions, e.g. 1 for the s-occupation in sodium, 5/3 for the p-occupation in fluorine.

v list

With this command you can remove an orbital occupation, if you specified a wrong one. list is again a list of shell indices in usual syntax.

&

This command has a different meaning in this menu than in the rest of define. Here it will repeat the extended Hückel calculation (perhaps you want to change some Hückel parameters for the next one).

*

* will not bring you back to the occupation numbers menu, but will terminate the whole occupation number and start vector section and will bring you to the last main menu, which is described in Section 4.4. If you want to leave this menu without assigning all electrons in your molecule to shells, define will issue a warning and suggest to continue defining occupation numbers. You can ignore this warning, if you do not want to assign all electrons.

e

Calculates and displays the extended Hückel total energy of your molecule.

f

f will give you some information about the commands in this menu.

You may overwrite occupation numbers once given by just redefining the corresponding shell. For example, if you choose shells 1-10 as closed shells and afterwards shell no. 9 as open shell (with any occupation number), the open shell will be correctly assigned.