MOLCAS manual:

Subsections

• 6.24.1 GRID_IT - Basic and Most Common Keywords

6.24 GRID_IT: A Program for Orbital Visualization

GRID_IT is an interface program for calculations of molecular orbitals and density in a set of cartesian grid points. Calculated grid can be visualized by separate program LUSCUS in the form of isosurfaces.

GRID_IT generates the regular grid and calculates amplitudes of molecular orbitals in this net. Keywords Sparse,Dense, Npoints specify the density of the grid. And keywords ORange (occupation range), ERange (energy range), Select allow to select some specific orbitals to draw.

As default GRID_IT will use grid net with intermediate quality, and choose orbitals near HOMO-LUMO region. Note, that using keyword All - to calculate grids for all orbitals or Dense - to calculate grid with very high quality you can produce a very huge output file.

GRID_IT requires the communication file RUNFILE, processed by GATEWAY and any formated INPORB file: SCFORB, RASORB, PT2ORB, generated by program SCF, RASSCF, or CASPT2, respectively. The output file M2MSI contains the graphical information.

Normally you do not need to specify any keywords for GRID_IT: the selection of grid size, as well as the selection of orbitals done automatically.

An input example for GRID_IT is:

  &GRID_IT
Dense
*  compute  orbitals  from  20  to  23  form  symmetry  1  and  orbital  4  from  symmetry  2
SELECT
1:20-23,2:4

GRID_IT can be run in a sequence of other computational codes (if you need to run GRID_IT several times, you have to rename grid file by using EMIL command, or by using keyword NAME)

&GATEWAY
  ...
&SEWARD
&SCF
&GRID_IT
NAME=scf
&RASSCF
&GRID_IT
NAME=ras
or, you can run GRID_IT separately, when the calculation has finished.

&GATEWAY
&GRID_IT
FILEORB=/home/joe/project/water/water.ScfOrb

This is quite important to understand that the timing for GRID_IT, and the size of generated grid file depends dramatically on the targeting problem. To get a printer quality pictures you have to use Dense grid, but in order to see the shape of orbitals - low quality grids are much more preferable.

The following table illustrates this dependence:

C₂₄ molecule, 14 orbitals.

Keywords	Time (sec)	filesize	picture quality
Dense, ASCII	188	473 Mb	best
Dense	117	328 Mb	best
Dense, Pack	117	41 Mb	below average
Default (no keywords)	3	9 Mb	average
Pack	3	1.4 Mb	average
Sparse	1.3	3 Mb	poor
Sparse, Pack	1.3	620 Kb	poor

6.24.1 GRID_IT - Basic and Most Common Keywords

Keyword	Meaning
ASCII	Generate the grid file in ASCII (e.g. to transfer to another computer), can be only used in combitation with NoLUSCUS
ALL	Generate all orbitals
SELECT	Select orbitals to compute