| symm_to_cell_const {cry} | R Documentation |
Cell parameter constrains from symmetry
Description
This function returns a set of constrains, as string character expressions, imposed by the specific symmetry group on the given unit cell.
Usage
symm_to_cell_const(SG)
Arguments
SG |
A character string indicating the extended Hermann-Mauguin symbol for the space group. |
Details
Space group symmetry imposes certain constraints on the values that unit cell parameters can take. For example, the symmetry represented by the monoclinic space group of extended Hermann-Mauguin symbol "P 1 2 1" is compatible with a unit cell in which alpha=gamma=90.
There is just a handful of constrains for unit cells. Here they are indicated with the following set of specific strings:
-
'No constrains' Like in a triclinic cell.
-
'alpha=90' The alpha angle is fixed at 90 degrees.
-
'beta=90' The beta angle is fixed at 90 degrees.
-
'gamma=90' The gamma angle is fixed at 90 degrees.
-
'gamma=120' The gamma angle is fixed at 120 degrees.
-
'alpha=beta=gamma' The three angle have the same value, different from 90 degrees.
-
'a=b' Cell side a is equal to cell side b.
-
'a=b=c' The three cell sides are equal.
Value
vcons A character vector. Each component is a string, like 'alpha=90' or 'a=b', that describes the type of constrain to be applied to a unit cell of a crystal structure with given space group symmetry (see above).
Examples
# P 1 1 2 (group number 3) corresponds to setting 2
SG <- translate_SG(3,set=2)
# Constrains for this symmetry
stmp <- symm_to_cell_const(SG)
print(stmp)
# R 3 (rombohedral setting)
stmp <- symm_to_cell_const("R 3 :R")
print(stmp)