Keyword GUESS

This keyword specifies the SCF starting density.
TB A tight-binding SCF starting density is calculated. This is the default.
CORE The SCF starting density is obtained from diagonalization of the core Hamiltonian.
HBOMD The CORE start density is used in each BOMD step.
FERMI The starting density is obtained by quenching a fractionally occupied SCF solution to integer occupation numbers.
RESTART The starting density is read from the restart file deMon.rst.
PROJECTED A projected starting density is requested.
ONLY The program stops after the generation of the starting density.
A good choice of the starting density can be crucial for the SCF convergence. In most cases, the tight-binding starting density is the recommended choice. For metal clusters, the core starting density may sometimes be advantageous. In Born-Oppenheimer molecular dynamics (BOMD) simulations the converged SCF density of the previous MD step is used by default as start density for the next MD step. With the GUESS option HBOMD a core start density is requested for each BOMD step. This decouples the SCF solutions of the individual BOMD steps. Note that this choice will increase computational demand. For the definition of the core Hamiltonian recall Section 1.4.

If the molecular orbitals of the start density (for the printing of the MOs see Section 4.12.2) exhibit only a very small HOMO-LUMO gap, the FERMI option may be used to obtain a better starting density. It should be noted that this option implies an SCF calculation and, therefore, is much more time consuming than the other starting density options. If a FERMI starting density is used, the OMA option of the keyword MIXING (see 4.5.6) should not be applied! The starting density can also be read from the restart file deMon.rst of a previous run in form of MO coefficients. Note that the restart file always contains canonical MO coefficients. With the keyword MOEXCHANGE (see 4.4.3), the molecular orbital ordering of the starting density can be altered. With the option PROJECTED MO coefficients from a different (usually smaller) basis set can be read from the restart file and used as start density. This can be a good strategy to accelerate SCF convergence for very large basis sets.

The option ONLY stops the program after the starting density is generated and written to the restart file. This option is recommended if a FERMI starting density was requested or the starting density needs to be altered after inspection. The starting density can be printed with the MAX=1 and MOS option of the PRINT keyword (4.12.2).