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##

Roothaan Parameters

In open-shell calculations within the restricted Hartree-Fock ansatz
(ROHF), the coupling between the closed and the open shells must be
specified using two parameters `a` and `b`, which depend
on the type of the open shell, the number of electrons in it (the
electron configuration), but also on the state to be calculated. For
example, there are three states arising from the *s*^{2} *p*^{2}
configuration of an atom (^{3}P, ^{1}D, ^{1}S) which have different
values of *a* and *b*. For the definition of these parameters and
their use refer to Roothaan's original paper [25].
For simple cases, `define` sets these parameters automatically. If
not, you have to enter them yourself. In this case, you will get the
following message:
ROOTHAAN PARAMETERS a AND b COULD NOT BE PROVIDED ...
TYPE IN ROOTHAAN a AND b AS INTEGER FRACTIONS
OR ENTER val FOR AN AVERAGE OF STATES CALCULATION
OR ENTER & TO REPEAT OCCUPATION NUMBER ASSIGNMENT

Note that *not* all open shell systems can be handled in this
way. It is possible to specify *a* and *b* for atomic calculations
with
*s*^{n}, *p*^{n}, *d*^{1}, and *d*^{9} configurations and for
calculations on linear molecules with *π*^{n} and *δ*^{n}
configurations. Furthermore, it is possible to do calculations on
systems with half-filled shells (where a=1, b=2). In the
literature you may find tabulated values for individual states
arising from *d*^{n} configurations, but these are **not**
correct. Instead, these are parameters for an average of all
states arising from these configurations. You can obtain these
values if you enter `val`

on the above question. For a
detailed description see Section 6.3.

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TURBOMOLE