# 4.2.24. GUESSORB¶

## 4.2.24.1. Description¶

The GUESSORB program generates a start guess for orbitals. The file GSSORB is created containing these orbitals. They are also put to the runfile and their presence is automatically detected by the programs SCF and RASSCF if needed.

## 4.2.24.2. Dependencies¶

The GUESSORB program requires that the one electron file ONEINT as well as the communication file RUNFILE exist. These are generated by the program SEWARD

## 4.2.24.3. Files¶

Below is a list of the files that are used/created by the program GUESSORB.

### 4.2.24.3.1. Input files¶

GUESSORB will use the following input files: ONEINT, RUNFILE (for more information see Section 4.1.1.2).

### 4.2.24.3.2. Output files¶

GSSORB

GUESSORB orbital output file. Contains a start guess for orbitals.

RUNFILE

Communication file for subsequent programs.

MD_GSS

Molden input file for molecular orbital analysis.

## 4.2.24.4. Input¶

Below follows a description of the input to GUESSORB.

### 4.2.24.4.1. Keywords¶

PRMO

This keyword will make GUESSORB print the orbitals that are generated. On the next line an integer is to be specified that controls how much output you get, see below. On the same line you can optionally specify a floating point number that control how many orbitals are printed. Only orbitals with orbital energy less than this number will be printed, default is 5.0 au.

• 1 — Only occupation numbers and orbital energies are printed.

• 2 — As for 1 but with an additional sorted list of orbital energies.

• 3 — As for 2 but with orbitals printed in compact format.

• 4 — As for 3 but orbitals are printed in full format.

PRPOpulation

This keyword will print a Mulliken population analysis based on the assumptions guessorb make with regards to populating orbitals.

STHR

This keyword controls how many orbitals will be deleted. On the next line you specify a threshold that have the default $$10^{-9}$$. The overlap matrix is diagonalized and only eigenvectors with eigenvalues larger that this threshold will be used, the other will be deleted. This removes near linear dependence.

TTHR

This keyword controls how many orbitals will be deleted. On the next line you specify a threshold that have the default $$10^6$$. The kinetic energy matrix is diagonalized in the space of virtual orbitals and only orbitals with energies below this threshold is used, the other will be deleted. This removes degrees of freedom describing core correlation.

GAPThr

This keyword controls how guessorb attempt to populate the orbitals. On the next line a threshold is specified that have the default 0.01. Using this threshold guessorb will make a closed shell configuration if it can find a HOMO/LUMO gap that is larger than the specified threshold. If that can not be done, guessorb will place a number of orbitals in an active space in such a way that the gap between the three spaces (inactive, active and secondary) will be larger than the threshold.

END of input

### 4.2.24.4.2. Input examples¶

In this example GUESSORB is used to produce a Mulliken population based on assumptions that are made for population of orbitals.

&GUESSORB
PrPopulation


In this example GUESSORB is used to construct an active space for RASSCF if there are near degeneracies around the fermi level. The orbital file that is produced can be fed directly into RASSCF without specifying the active space.

&GUESSORB
GapThr = 0.5

&RASSCF
LumOrb