The Coulomb and XC (exchange + correlation) potentials are computed from the fit approximation of the charge density (see Chapter 1.2).
The fit coefficients of this approximation for the first SCF cycle, needed to compute the first Fock matrix, are read from the fragment files: the start-up density is chosen as a sum-of-fragment-densities (fit approximations) and this combined density defines the initial potential.
In the SCF restart run the fit coefficients may be read from the attached TAPE21 file, see the key RESTART.
In some applications you may want to modify the initial fit coefficients (from the restart file or the fragment files). This is achieved with the key MODIFYSTARTPOTENTIAL. It allows you to scale them in some way so as to represent user-chosen amounts of spin-α and spin-β fit density on some or all of the fragments. This will adjust the spin-α and spin-β initial potentials.
This option applies only to unrestricted calculations of course. It may be used to help the program find a particular state. This might, for instance, be hard to find otherwise due to the a-b symmetry in the start-up situation. It may also be useful to speed up the SCF convergence in case you know what the final distribution of spin-α and spin-β density over the molecule will approximately be.
A general key: it has an argument or a data block.
Must be two numbers, ASPIN and BSPIN, if provided
at all. They specify the (relative) amounts of spin-α and spin-β fit density to define the spin-dependent
potential at the first SCF cycle.
The coefficients retrieved from the fragment files (or from the restart file in
case of a SCF restart) are scaled
accordingly. This will not affect the total amount of fit density: the absolute values of ASPIN and BSPIN play no role,
only their ratio.
In case of a restart run the restart file must have been generated in a restricted calculation, while the continuation run must be an unrestricted one.
If no argument is given a data block must be
supplied with records frag alfa // beta.
This is very much similar to the main option with ASPIN and BSPIN: you specify ASPIN and BSPIN now separately for each fragment. This involves somewhat more input but increases the possibilities to tune the initial potential. Again this can be applied only in an unrestricted calculation. It cannot be used in a restart: the affected fit coefficients are those from the fragment files, while in an SCF restart run these are ignored and replaced by the coefficients on the TAPE21 restart file.
Each line specifies a frag with its corresponding ASPIN and BSPIN fit partitioning. If frag is the name of a fragment type, the specified ASPIN-BSPIN is applied to all individual fragments of that type. Alternatively an individual fragment can be specified, using the format fragtype/n, where n is an index between one and the total number of fragments of that type. In such a case the ASPIN-BSPIN data applies only to that particular fragment while different values may be supplied for the other fragments of the same type.
It is allowed to specify for certain fragment types individual fragments and for other fragment types only the type. Duplicate specifications are not allowed; an individual fragment must not be specified if its fragment type is also specified as a whole.
If the data block form is used, only the fit coefficients of the referenced fragments are affected. For the not-referenced fragments the fit densities are used as they are defined on the corresponding fragment files.
The SCF convergence of a spin-unrestricted calculation usually improves when you start with potentials that correspond to the correct ratio of spin-α and spin-β electrons. By default ASPIN=BSPIN=0.5, as implied by the spin-restricted start density of the fragments or restricted molecule.
The total amount of fit density used on the first iteration is defined by the sum-of-fragment densities (or the density on the restart file). This may be different from the total nr. of electrons in the actual calculation. On the second SCF cycle the fit density will internally be normalized so as to represent the correct number of electrons.
The block-form of the key makes the start up of broken symmetry calculations easy. For example one may want to start a calculation in broken symmetry with spin-α density on one fragment and spin-β density on another, e.g. in a spin-unrestricted calculation of H2 at large separation. It is particularly useful for larger systems, e.g. for magnetic coupling between spin-polarized magnetic centers, as in Fe-S complexes : start with oppositely polarized Fe centers, but with, for instance, the remaining bridge and terminal ligands unpolarized. See also the N2+ sample run in the examples.