CASINO manual - Theory of Condensed Matter

The clearup twistav script can be used to clear up the output from a pwscf twist-averaging run.
Default behaviour of twistav pwscf (on all machines):
Note: in the following ntwist is 12, or the value of the optional argument --ntwist, while xwfn.data
refers to whatever wave function file is specified in the pw2casino.dat file (either bwfn.data.b1
[default], bwfn.data or pwfn.data).
For a complete twist-averaging run, the following steps are performed in sequence:
(A) pwscf generates ntwist xwfn.data.$ files, where $ is a sequence number from 1 to ntwist. Each
succeeding run will have a different twist.
(B) casino runs a full VMC or DMC run on each of the xwfn.data.
On batch queue systems, twistav pwscf will by default do two batch script submissions, the first—
handled by the runpwscf script—executing step (A), and the second—handled by the runqmc script—
executing step (B).
In principle, this wastes some unnecessary time (the time spent waiting for the QMC batch script to
start) but this is unavoidable if twistav pwscf uses separate runpwscf and runqmc scripts to handle
the DFT xwfn generation and QMC calculations. This may be changed in the future, if anyone can
be arsed.
Note that all calculations will be done on the number of cores requested on the command line (with
the --nproc/-p flag) irrespective of whether they are DFT or QMC calculations. You may override
this for the DFT calculations by using the --nproc dft flag to twistav pwscf.
Modifications to default behaviour (on all machines)
(1) twistav_pwscf --dft_only : execute only step (A), generating ntwist
xwfn.data.$ files.
(2) twistav_pwscf --qmc_only : execute only step (B).
This requires that the ntwist xwfn.data.$ files already exist; if they don’t the script will whinge and
die.
(3) twistav_pwscf --startqmc=M : Start the chain of QMC runs with file xwfn.data.M .
Modifications to default behaviour (batch machines only)
On batch machines, there is an additional complication due to the walltime limits on particular queues
which may require full twist-averaging runs to be split into sections. The following method may be
used to do this.
(4) twistav_pwscf --splitqmc=N : Split step B into N separate batch script
submissions. If no value is supplied [--splitqmc] the run will be split
into two.
Example: ntwist=13, and twistav pwscf --splitqmc=4 will result in four step B batch submissions
with 3, 3, 3, 4 twists.
Note there is no facility for splitting step A into sections, i.e., all DFT wave function generation runs
will always be run in a single batch script submission. This is because we assume the DFT runs are
fast and you have adequate job time limits. If this is not the case then simply do multiple sets of
twistav pwscf runs.
29 Finite-size correction to the kinetic energy
29.1 Finite-size correction due to long-ranged correlations
Consider a periodic system of N particles. Suppose there are N s species present, each with mass m α
and charge q α , and let N α be the number of particles of type α. Let r iα be the position vector of the
ith particle of type α. Let the simulation supercell have volume Ω.
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