In quantum espresso, 'alat' as written in the documentation is the 'lattice constant' of the system. So, far this is true for cubic system and it won't affect either we input celldm(1) or A, B, C and cosine of angles as they will be the same (lattice constant). My confusion is for other than cubic system. Say in orthorhombic system; clearly, the lattice constant is not A only. We must specify A, B, C and the angles. Here A, B and C can be different. But still the code says 'alat' is equal to celldm(1) or 'A' in this case. But this is not true right? I am confused here. What is the meaning of 'alat' or 'lattice constant' in Quantum Espresso? Can someone explain it in an orthorhombic crystal system and not in a cubic system?
celldm(i), i=1,6 are crystallographic constants - see the ibrav variable.
Specify either these OR A,B,C,cosAB,cosBC,cosAC NOT both.
Only needed values (depending on "ibrav") must be specified
alat = celldm(1) is the lattice parameter "a" (in BOHR)
If ibrav==0, only celldm(1) is used if present;
cell vectors are read from card CELL_PARAMETERS
(source: from the documentation)
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