CASINO manual - Theory of Condensed Matter

Detailed information about each of the backflow functions can be found in Sec. 23.
Notes:
• The polynomial form of η requires the specification of an expansion order as shown above. The
two lines have to be removed when using any of the other two forms.
• The η function has separate cutoff lengths for each of the spin-dependencies. The first one must
be specified; every cutoff length not listed is initially set to the value of the first one. Set the
‘Optimizable’ flag to 2 to constrain all cutoff lengths to be the same during optimization too.
• ‘Type of e-N cusp conditions’ in µ and Φ is used to specify whether to regard the atoms in the
set as all-electron (AE) or pseudo-atoms (PP). PP cusp conditions are less restrictive than AE
ones. The user should set this flag to 0 only if a pseudopotential is being used for the atom in
question.
• ‘Apply no-curl constraint’ in Φ is used to reduce the number of parameters in Φ and Θ when
this number is too large for optimization to succeed. In principle, this flag should always be set
to zero for the best results.
• It is recommended that vm filter be set to T in the input file when optimizing backflow parameters
using unreweighted variance minimization.
• The AE CUTOFFS section will appear automatically in systems with all-electron atoms, and all
cutoffs will be flagged as optimizable by default.
7.4.4 Excitations and multideterminant-expansion coefficients
The xwfn.data file contains data that define the orbitals produced by the wave-function generating
code. The file also specifies a reference configuration (the form of the Slater wave function), which
may consist of one or more determinants. The ‘MDET’ block in the correlation.data file allows
one either to use the reference configuration or to specify excitations, additions or subtractions from
the reference configuration. Furthermore, it enables the user to construct an expansion in several
determinants with optimizable expansion coefficients. The MDET block consists of a title followed
by an arbitrary number of lines (up to ‘END MDET’), which contain tokens used to specify the
excitations and the multideterminant configuration.
The simplest case is when we want to use exactly the reference configuration specified in xwfn.data
(this is the default behaviour if no MDET block is supplied). In this case, the single token ‘GS’ is
used:
START MDET
Title
MDET example: use reference configuration in xwfn.data file.
Multideterminant/excitation specification (see manual)
GS
END MDET
Promotions, additions and subtractions of electrons are specified using the keywords ‘PR’, ‘PL’ and
‘MI’, respectively. Several changes to the reference configuration can be made at once. Here is an
example of a single-determinant (‘SD’) excited-state calculation:
START MDET
Title
MDET example: use a single-determinant excited state.
Multideterminant/excitation specification (see manual)
SD
DET 1 2 PR 3 4 5 6
DET 1 2 PL 6 3
DET 1 2 MI 6 3
END MDET
Notes:
• ‘DET 1 2 PR 3 4 5 6’ means ‘in determinant 1, spin 2 (down), promote an electron from band
3, k-point 4, to band 5, k-point 6’.
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