CASINO manual - Theory of Condensed Matter

to |p x | in the limit of perfect sampling (i.e., a biased estimate with finite sampling). You will
also get larger error bars on |p| than |p| x
. See Sec. 33.8 for more details and for an example see
J. Chem. Phys. 127, 124306 (2007).
DMC AVE PERIOD (Integer) Number of consecutive local energies that are averaged together in
DMC before writing them to the dmc.hist file. The only effect of this keyword is reduce the
number of lines in dmc.hist by a factor of 1/dmc ave period. Note that if dmc equil nstep
or dmc state nstep are not divisible by dmc ave period, they will be rounded up to the
nearest integer multiple of it.
DMC DECORR PERIOD (Integer) Length of the inner decorrelation loop in DMC. The algorithm
will perform dmc decorr period configuration moves between successive evaluations of
expectation values other than the energy. Setting dmc decorr period to a value greater than
1 should reduce the serial correlation of the data. Notice that dmc decorr period differs from
its VMC counterpart in that in DMC local energies are calculated at intermediate steps (they
must), and these additional values are averaged into the energy data. Therefore, for calculations
which do not require expectation values other than the energy, changing dmc decorr period
from 1 to some value x is equivalent to setting multiplying both dmc equil/stats nstep and
dmc ave period by x. In a preliminary DMC calculation, dmc decorr period specifies the
frequency with which configurations are written out.
DMC EQUIL FIXPOP (Real) If VMC and DMC energy are too different, the population increases
during the initial phase of equilibration before the reference energy can counteract. This parameter
(between 0.0 and 1.0) specifies an initial fraction of the equilibration phase during which
the population and total weight are fixed to the target weight. Setting this parameter to e.g.
0.5 will prevent such explosions and should have negligible effect on equilibration time.
DMC EQUIL NBLOCK (Integer) Number of blocks into which the DMC equilibration phase is
divided (if dmc equil nstep is not divisible by dmc equil nblock, then the number of steps
will be increased to the nearest multiple of the number of blocks). Note that having multiple
blocks does not increase the amount of data collected, merely the frequency with which data is
written to files; the final answer should be essentially the same, irrespective of the number of
blocks. Specifically, at the end of each equilibration block, the following significant actions are
performed:
(1) Write processor- and config-averaged data to dmc.hist (one line for each step in the current
block).
(2) Print monitoring data to the out file (block-averaged quantities).
(3) Make a backup copy of the config.out file (if catastrophe protection is turned on with the
dmc trip weight keyword).
(4) Write the dmc.status file.
(5) Write the current state of the system, and all configurations in the current population to the
config.out file (note that by setting the checkpoint keyword to 0, this step can be skipped
until the end of the final block, or skipped completely if checkpoint=-1, but this is not the
default).
Note that if accumulating expectation values other than the energy, data is not written to the
expval.data file after each block, as it would be during the statistics accumulation phase.
Also, having too many blocks will make the code slower, and if the run is not massively long it
is perfectly in order to have only one DMC equil block (which is the default).
DMC EQUIL NSTEP (Integer) Number of DMC steps performed on each processor in the DMC
equilibration phase, and consequently, the total number of local energy samples (averaged over
configurations and processors) written to the dmc.hist file. The equilibration phase may be
partitioned into dmc equil nblock blocks, but this does not affect the total number of steps
(just how frequently stuff is written out). However, if dmc equil nstep is not divisible by the
number of blocks, then it will be rounded up to the nearest multiple of dmc equil nblock.
Furthermore, dmc ave period consecutive local energies may be averaged together in DMC
before writing them to the dmc.hist file (hence reducing its size), but again, if dmc equil nstep
is not divisible by dmc ave period, it will be rounded up to the nearest multiple of it. Note the
difference in parallel behaviour compared to vmc nstep, which is not a per processor quantity;
this is because the DMC phase is parallelized over configurations.
DMC INIT EREF (Physical) If set, dmc init eref defines the initial reference energy for a DMC
calculation. If unset, the VMC energy is used instead (default). This keyword is ignored if the
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