CASINO manual - Theory of Condensed Matter

for all-electron calculations scales rather badly with atomic number Z; this scaling is improved by
using pseudopotentials.
casino reads pseudopotential data from a file called xx pp.data, where xx is the chemical symbol
of the element in lower-case letters. The file contains a logarithmic radial grid and the values of the
different angular momentum components of the pseudopotential at each grid point.
On the rare occasions when you might want to use two or more different pseudopotentials for atoms
with the same atomic number (say in a surface, and in an atom or molecule absorbed on that surface),
then you may use additional pseudopotentials renamed as, e.g., xx2 pp.data. Different types of
pseudoatom are flagged in xwfn.data by adding multiples of 1000 to the original atomic number,
e.g., atomic numbers 12, 1012 and 2012 correspond to atoms using pseudopotentials mg pp.data,
mg2 pp.data and mg3 pp.data, etc.
A library of pseudopotentials suitable for use with casino is available at:
http://vallico.net/casinoqmc/pplib/
An important point is that exactly the same pseudopotential should be used in the DFT/HF calculation
that generates the trial wave function and in casino. Other programs do not in general understand
the casino pseudopotential format, and so the information must somehow be transformed so
that they do.
For programs using Gaussian basis sets such as gaussian9X/0X and crystal, the pseudopotential
must be re-expanded in Gaussian functions multiplied by powers of r. If you are using the Cambridge
pseudopotentials, such expansions (in formats suitable for these two programs) are included in the
on-line library. There is a casino utility—ptm—which can manipulate pseudopotential files on grids
and their Gaussian expansions in various useful ways.
As for plane-wave programs, castep understands the casino grid format and can read such
files directly. Other plane-wave programs require conversion utilities, which are included in the
~/CASINO/utils/pseudo converters/ directory.
For atomic calculations on radial grids, awfn.data files generated with the Cambridge pseudopotentials
are available in the on-line library.
6.1.3 The input file
Having prepared a trial wave-function file and (perhaps) a pseudopotential file, you need to tell casino
exactly what to do with them. casino takes its instructions from a file called input, which contains
a flexible list of keywords. These control the behaviour of the calculation, switch on and off various
options and so on. Take a moment to examine the various input files in ~/CASINO/examples/ to get
a feel for what they look like.
A complete list of input keywords, together with their definitions, is given in Sec. 7.3. Further details,
including default values, may be found by using the casinohelp utility. This tends to be more up to
date than the manual, since it interrogates casino directly. Type casinohelp all to get a list of all
keywords that casino knows about, or casinohelp keyword for detailed help on a particular keyword.
Type casinohelp search text to search for the string ‘text’ in all the keyword descriptions.
Although there are many keywords, the beginner can play around by changing only a few of them.
Here’s a (very) rough guide. Advice on good values to use will be given in the subsequent sections
explaining how to do VMC, DMC and optimization calculations.
• General (system-dependent) keywords:
NEU, NED Number of electrons of up and down spin;
PERIODIC Whether the system is periodic or not;
NPCELL Array of primitive cells making up the simulation cell (not required for finite systems).
• Other vital keywords:
RUNTYPE Type of QMC calculation: ‘vmc’, ‘vmc dmc’, ‘vmc opt’, etc.;
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