Electronic Configuration
************************

Not specifying occupation numbers in the input will *not*
automatically result in the computation of the ground state. It may
even lead to non-convergence in the SCF and/or in the determination of
minimum-energy geometries or transition states. Therefore: whenever
possible, specify occupation numbers explicitly in the input (key
OCCUPATIONS)!

Misunderstanding results of a calculation may easily result from a
lack of awareness of how ADF treats the electronic configuration,
which orbitals are occupied and which are empty. Unless you specify
occupation numbers in the input, they will be determined from the
Aufbau principle but only during the first few SCF cycles. Thereafter
the distribution of electrons over the different symmetry
representations is frozen (see the key OCCUPATIONS, options AUFBAU and
aufbau2). If at that point the potential has not yet sufficiently
relaxed to self-consistency the *final* situation may be non-Aufbau.

A related aspect is that the *ground state* does not necessarily
*have* an Aufbau occupation scheme. In principle, different competing
electronic states have to be evaluated to determine which has the
lowest total (strongest bonding) energy.

*Check output always carefully as to which orbitals are occupied.* In
general, whenever possible, supply occupation numbers in the input. Be
aware that the automatic choice by the program may, in a geometry
optimization, result in different configurations in successive
geometries: the automatic assessment by the program will be carried
out anew in each SCF procedure. If competing configurations with
comparable energies have different equilibrium geometries, the
geometry optimization has a high failure probability. The gradients
computed from the SCF solution of a particular configuration drive the
atoms in a certain direction, but in the next geometry, when the
program re-determines the occupations and finds a different
configuration, the resulting gradients may drive the atoms in another
direction.

See the keys CHARGE and OCCUPATIONS for user control of occupation
numbers.


Spin-unrestricted versus spin-restricted, Spin states
=====================================================

If your molecule has unpaired electrons, you should run an
unrestricted calculation, in principle. However, if this exhibits
convergence problems (or if you simply want to save time: an
unrestricted calculation requires about twice as much CPU time and
data storage), you may consider doing it in two steps. First, run a
spin-*restricted* calculation. Then perform a spin-unrestricted
calculation using the restricted adf.rkf as a restart file. In the
follow-up calculation you should specify the precise occupation
numbers for the state you're interested in, *and* use the SCF input
key to specify *only one* SCF cycle (iterations=1). This prohibits
convergence (so you keep the converged *restricted* orbitals) and
gives you a fairly adequate approximation to a converged unrestricted
result. See also the H2 example run for a discussion in the Examples
document.

An unrestricted calculation does not necessarily yield the multiplet
configuration (triplet, doublet, ...). This is a rather complicated
matter; see the discussion on multiplet states, key
SLATERDETERMINANTS.
