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10 Density functional calculations

MOLPRO is able to carry out standard Kohn-Sham DFT calculations using either the spin-restricted (ks or rks) or spin-unrestricted (uks) formalisms. Here is a simple example, which does a local-density approximation (LDA) calculation for formaldehyde.

***,formaldehyde
geometry={
C
O  , C , rco
H1 , C , rch , O , hco
H2 , C , rch , O , hco , H1 , 180
}

rco=1.182 Ang
rch=1.102 Ang
hco=122.1789 Degree

rks

The exchange-correlation functional, and associated potential, are integrated numerically, and in principle it is necessary to supply parameters to allow the construction of an appropriate grid. However, the grid is built using a model adaptive scheme, and it is normally necessary to specify only the required energy accuracy. This threshold is in turn taken by default from the overall energy convergence threshold, so in most calculations it is not normally necessary to specify anything at all.

Many different exchange and correlation functionals are contained in the program. They are implemented in a modular fashion, with both computer code and documentation being built directly from their mathematical definition. This means that you can always find the precise definition of a functional in the user manual. Each functional has a keyword which is used to identify it in the input file. The functionals to be used are given one after each other as options to the ks command; for example, the following does a Kohn-Sham calculation using Becke's exchange functional, and the Lee-Yang-Parr correlation functional.

ks,b,lyp

Another commonly used combination (in fact, the default) is ks,s,vwn which gives LDA (Slater-Dirac exchange, Vosko-Wilk-Nusair correlation). Finally, the program is aware of some combinations of functionals, for example ks,b3lyp which is the hybrid B3LYP functional consisting of weighted combinations of various density functionals together with a fraction of exact (Hartree-Fock) exchange.

Other options, such as the closed- and open-shell orbitals and the wavefunction symmetry can be defined as explained in section 4.3.



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molpro@molpro.net 2017-12-17