Chemical substitution for structure optimization

Variations of chemical concentrations at given sites during the search must be specified in a block starting with the

= OCC_DELTA

header flag, followed by a list of all elements and the range of their concentrations. If the total site occupation does not sum to one at any point, this is implicitly interpreted as a vacancy. The command follows a slightly different syntax than those for the Geometrical displacements or Vibrational amplitudes.

Syntax

See also the example below

POSCAREL(_site) number(s) = chem1 start end step, chem2 start end step, chem3 start end step, chem4 start end step, chem5 start end step
POSCAREL(_site) number(s) = fix1 chem1 + fix2 chem2 start end step, chem3 start end step, chem4 start end step, chem5 start end step

where

POSCAREL is the POSCAR element and not the chemical species defined via ELEMENT_MIX. See also element name collision.
site is optional, and has the same functionality as the one in the Geometrical displacements.
number(s) also behave as in the Geometrical displacements.

On the right-hand side of the = sign, blocks are included in a comma-separated list. Each list element can have the two possible forms:

chem start end step
fix1 chem1 + fix2 chem2 (+ ...) start end step

where chem* is one of the chemical elements that you have defined via ELEMENT_MIX or the special flag Vac (not case sensitive) for vacancies. Notice that a maximum of five distinct chemical elements (including vacancies) can be used at each atomic position.

start, end, step, and fix* are floating point numbers between 0 and 1 defining the fractional occupations in percent.

When using the first syntax, the program will search all independent combinations of chem* in the range specified. The second syntax allows you to fix the relative concentration of (at least two, at maximum five) elements via the numbers fix*, creating a “combined atom”, whose concentration with respect to the other chemical species specified in the following blocks will be varied according to the start, end and step values defined afterwards. When you combine chemical species with fixed concentration, the fix* numbers will give the ratio between those chemical species, while the overall concentration of the “combined atom” is still given by start end step. The total number of chemical species on each site must be at most five.

Example

= OCC_DELTA
! Concentration of oxygen atom 1 (and symmetry-equivalent atoms) will
! be varied from 80% to 100% with 5% steps (rest: vacancies)
O 1 = O 0.8 1.0 0.05

! Occupation of M_top sites (M is a POSCAR element) will be varied
! between 40% iron + 60% nickel and 60% iron + 40% nickel, with 5% steps.
M_top = Fe 0.4 0.6 0.05, Ni 0.6 0.4 0.05

! As above, varying Fe 30-50%, Ni 60-40%, while at the same time
! keeping Ti constant at 10%
M_top = Fe 0.3 0.5 0.05, Ni 0.6 0.4 0.05, Ti 0.1

How atoms are addressed on the left is described on the main DISPLACEMENTS page. Note that if you want to vary the concentrations of multiple elements, the number of steps in the ranges must be the same, and none of the steps must have a total occupation greater than 1. To keep an element at a constant concentration while varying others, you can assign that concentration using only one number instead of three, as in the example for Ti above. Alternatively, the same input format with of start end step with start == end and arbitrary step is also interpreted as a constant concentration.

In the OCC_DELTA block, the element on the left must be the element as defined in the POSCAR file, and the elements on the right must be chemical elements, defined either by ELEMENT_MIX or ELEMENT_RENAME in the PARAMETERS file.

Note that a maximum of five distinct chemical elements (including vacancies) can be used at each atomic position.

For some applications, it can be useful to apply a static offset, without re-doing the reference calculation. For this purpose, the OCC_DELTA block also accepts single-value input (per element) on the right:

= OCC_DELTA
O 1 = O 0.8                         ! Concentration of oxygen atom 1 (and symmetry-equivalent atoms) will be fixed to 80% (rest: vacancies)
M_top = Fe 0.6, Ni 0.4              ! Occupation of M_top sites (M is a POSCAR element) will be fixed to 60% iron + 40% nickel.

Note

Due to the Fortran format currently used, start, end, and step will be truncated at the second decimal digit by rounding (i.e., 85.263 -> 85.26, while 85.265 -> 85.27).
As for the Geometrical displacements and for the Vibrational amplitudes, the concentration steps above will be applied to all symmetry-equivalent atoms, unless you turn off symmetry via SYMMETRY_FIX or The SYM_DELTA tag.
The minimum number of blocks is one. You can use this to specify a fixed chemical substitution on the atomic site, which can differ from the one you specified in the VIBROCC file. This is generally not recommended.