CASINO manual - Theory of Condensed Matter

DTDMC (Real) Time step for DMC run (atomic units). The DMC time step must be small, as
the DMC Green’s function is only exact in the limit of zero time step: see Sec. 13. Typically
the DMC time step is about two orders of magnitude smaller than the VMC time step, and
the DMC move-acceptance ratio should be about 99.9%. For accurate work, one must always
investigate time-step bias by plotting the DMC energy against the value of dtvmc. Provided
the time step is sufficiently small that the root-mean-square distance diffused by each particle at
each step is much less than the shortest physically relevant length scale, one can expect to find
the time-step bias in the DMC energy to be linear; hence it is straightforward to extrapolate
to zero time step. The extrapolate tau utility exists to help with the extrapolation to zero
time step.
DTVMC (Real) Time step for VMC run (atomic units). The form of the VMC transition-probability
density used by casino is discussed in Sec. 12. As described in Sec. 12.4, dtvmc should be
chosen so that the overall move acceptance ratio is close to 50%. In most normal systems, the
appropriate value is between 0.1 a.u. and 0.6 a.u. If dtvmc is given a sensible starting value
(and, for normal systems, anything in this range is sensible), setting opt dtvmc to 1 will cause
the time step to be optimized automatically. For very low-density systems, a much larger value
of dtvmc is appropriate.
DTVMCS (Block) Use this keyword to specify a VMC time step for each particle family explicitly,
as well as to determine whether to optimize each of them individually. The contents of this
block override the values of dtvmc and opt dtvmc. One line is to be written for each ‘family’
of particles, the format of each line being:
DTVMC OPT DTVMC
where ‘DTVMC’ is the value of the time step, and ‘OPT DTVMC’ can be 0 or 1, indicating
whether to optimize the corresponding time step or not.
DTVMC SHIFT (Real) dtvmc shift is an optional shift in the VMC transition probability, which
can be used to ‘encourage’ electrons to be more mobile. dtvmc shift is expressed in units of
the square root of dtvmc.
E OFFSET (Physical) This keyword gives a constant shift E offset in the total energy per electron
such that the final result will be E = E calc − E offset . The default is zero. This allows the user
to add any constant contributions to the total energy that are not calculated within casino.
EBEST AV WINDOW (Integer) Averaging window for calculating the ground-state energy during
equilibration. During DMC equilibration the best estimate of the ground-state energy is taken
to be the average local energy over the last ebest av window moves. The default of 25 is
usually sufficient.
EDIST BY ION, EDIST BY IONTYPE (Block) The edist by ion block allows fine control
of the initial distribution of the electrons before equilibration starts. The standard algorithm
shares out the electrons amongst the various ions weighted by the pseudo-charge/atomic number
of the ion. Each electron is placed randomly on the surface of a sphere surrounding its parent
ion. There are certain situations, for example a simple crystal with a very large lattice constant,
where the standard algorithm in the points routine may give a bad initial distribution, which
cannot be undone by equilibrating for a reasonable amount of time. This keyword allows a
user-defined set of electron/ion associations to be supplied. The syntax is to supply N ion lines
within the block which look like, e.g., 1 4 4, where the three numbers are: the ion sequence
number; the number of up-spin electrons associated with this ion; the number of down-spin
electrons associated with this ion. Alternatively one may use the edist by iontype keyword
block, where you replace the ion sequence number with the ion type sequence number and the
information is supplied only for each particular type of ion.
EMIN MIN ENERGY (Physical) This keyword sets a minimum energy threshold for energy minimization,
used to reject low-quality wave functions which produce spurious low VMC energy
estimates. The value of emin min energy should (ideally) be set slightly below the groundstate
energy. The ground-state energy is often not known, in which case a good estimate can
sometimes be supplied. If this keyword is not set manually, casino will supply an automatic
guess derived from the preceding VMC run. See Sec. 25.3.6 for more details.
EMIN XI VALUE (Real) This keyword sets the value of the ξ parameter used to control semiorthogonalization
in energy minimization. It should rarely be changed by users. See Sec. 25.3
for details.
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