Electronic Configuration

Not specifying occupation numbers in input will not automatically result in the computational 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 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 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 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 takes a factor 2 more CPU time and data storage), you may consider to do it in two steps. First, run a spin-restricted calculation. Then perform a spin-unrestricted calculation using the restricted TAPE21 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 (triple, doublet ...). This is a rather complicated matter, see the discussion on multiplet states, key SLATERDETERMINANTS.