CCL: Summary: Integration grid defaulting to unpruned value

Many thanks.

My original question was


I am calculating the heats of reactions containing organo-mercury compounds. I am using G03 and DFT methods. The program, when mercury is involved, defaults to an unpruned integration grid for this atom. The manual states that one should use the same grid when comparing energies. Does this mean that I should be defining an unpruned grid for all of my calculations i.e. the compounds without mercury in the isodesmic reaction? If so how do I define an unpruned grid the same as the one being used by gaussian when mercury is involved. The manual does not talk about unpruned grids. I have enclosed the relevant output section below.

And the replies:-


   Only the mercury atom will use the unpruned grid.  All the other

atoms will use the default, pruned grid if it is defined.


   So you need not do anything special in generating the pieces of

the reaction.  Each element will be done the same way if you use

the defaults in Gaussian.


   The point of printing the informational message is in the event that

you compare with another program or a future release of Gaussian were

a pruned grid for mercury might be used.

Douglas J. Fox

Technical Support

Gaussian, Inc.





I think the manual is referring to using different grid densities, that

is, INT=ULTRAFINE versus INT=FINE. A pruned grid has some of the points

removed (hence the word 'pruned') so it is faster, but it should give

the same result as an unpruned grid, for the same integration grid



To summarise, I think you don't need to worry about the pruned grid, so

long as you use the same integration grid density (for example, the

default integration grid density) for all calculations.






Ian Hovell - Ph.D.


Centro de Tecnologia Mineral - CETEM

Ministerio da Ciência e da Tecnologia- MCT

Avenida Ipê, No 900 - Cidade Universitaria

Ilha do Fundão Rio de Janeiro RJ Brasil

CEP 21941-590

tel 00 55 (xx) 3865 7344 ou 3865 - 7216

Fax 00 55 (xx) 22602837 ou 2290-4286

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