CASINO manual - Theory of Condensed Matter

lattice points of the supercell offset by a constant vector k s in the first Brillouin zone of the simulation
cell.
In an explicitly correlated method such as QMC the need to describe long-range correlations forces
one explicitly to construct a simulation supercell consisting of several primitive cells. By contrast, in
single-particle methods such as DFT, the problem can be reduced to a single primitive cell with a grid
of k points, and the simulation supercell is never explicitly constructed.
You could generate a trial wave function for use in your QMC calculation by performing a DFT
calculation for the simulation supercell at a single k point. However this would be inefficient: the
translational symmetry within the simulation cell would not be exploited to reduce memory requirements
in both the DFT calculation and the subsequent QMC calculation. Instead you should generate
the trial wave function for a simulation supercell consisting of l × m × n primitive cells by performing
a DFT calculation in a single primitive cell with an l × m × n mesh of k points.
In summary, your DFT calculation should be set up to generate a set of orbitals for a single primitive
unit cell with an l × m × n k-point grid, whilst the QMC calculation should be set up for a simulation
supercell consisting of l × m × n primitive cells. In the input file, the number of up- and down-spin
electrons neu and ned should be appropriate for the supercell, whilst the npcell block, which specifies
the size of the supercell, should contain ‘l m n’.
Note that QMC methods cannot exploit symmetry (apart from time-reversal symmetry), and hence
a complete l × m × n grid of k vectors must be supplied in general. (How you do this depends on
your DFT code.) If you are using a grid of k vectors with time-reversal symmetry then your trial
wave function in the subsequent QMC calculations can be chosen to be real, and the complex wf
input keyword should be set to F. In this case you only need to supply one out of each ±k pair. If
you do not have time-reversal symmetry you must supply the complete grid of k vectors and set the
complex wf keyword to T.
Typically you will need to perform QMC calculations in two different supercell sizes and extrapolate
your results to the limit of infinite cell size. How exactly you proceed depends a good deal on what
you are calculating. The problem of finite-size errors in QMC calculations is discussed extensively in
Ref. [14].
6.6 How to run the code: RUNQMC
The runqmc script (which should be in your path after installing casino) is designed to reduce the
effort of doing QMC calculations to just entering one command. It is most useful when using parallel
machines with a batch queueing system. It is written to detect all common errors that a user may
make in setting up a calculation, so that the fault is detected immediately rather than when the batch
job starts (which may be many hours later). See the utils/runqmc/README file for more information.
runqmc can run calculations on single- and multiprocessor workstations, and on clusters. To run the
casino calculation set up in the current directory, simply type runqmc. This will automatically—and
possible a little dangerously—occupy all cores on a workstation, or the maximum permitted allocation
(in both number of cores and walltime) on a cluster. To change this, or to specify other calculation
parameters, there are various options available.
An example on a big parallel machine:
runqmc --nproc=224256 --walltime=10h --shm=12
will run the code in parallel using 224256 MPI processes (distributed among computational nodes
depending on the number of cores per node in the machine, which runqmc knows about) using shared
memory over groups of 12 processes with a walltime limit of 10 hours (which may affect what queue
your job is put in).
Note that this is equivalent to
runqmc -p 224256 -T 10h -s
(provided the machine has 12 cores per node) i.e., most options come with a longer version (with two
hyphens and an equals sign) and a shorter easier-to-type version (with one hyphen, and zero or more
spaces before the option value).
A really important point which trips quite a few people up: on machines with batch queue systems
everyone is used to manually creating batch scripts which tell the machine how to run the code, how
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