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hi,
I am confused about two lattice types listed in my OUTCAR file when performing simple optimization.
I get the following message:

============================
LATTYP: Found a simple monoclinic cell.
ALAT = 5.6160778470
B/A-ratio = 0.8791697070
C/A-ratio = 2.7189498063
COS(beta) = -0.5705551425

Lattice vectors:

A1 = ( 1.4035875445, -0.0000032229, -5.4378554954)
A2 = ( -0.0000273284, -4.9374855153, -0.0000107338)
A3 = ( -14.3199295447, -0.0000688885, 5.3016451527)
Subroutine PRICEL returns following result:

LATTYP: Found a triclinic cell.
ALAT = 6.9143096256
B/A-ratio = 1.8499548214
C/A-ratio = 0.7140966753
COS(alpha) = 0.0000119398
COS(beta) = 0.3570588556
COS(gamma) = 0.8449755946

Lattice vectors:

A1 = ( -6.4581859600, -2.4687795885, -0.0681105519)
A2 = ( -11.5127570472, -0.0000768846, -5.5740658655)
A3 = ( -0.0000273284, -4.9374855153, -0.0000107338)

2 primitive cells build up your supercell.


Analysis of symmetry for initial positions (statically):

Routine SETGRP: Setting up the symmetry group for a
simple monoclinic supercell.


Subroutine GETGRP returns: Found 2 space group operations
(whereof 2 operations were pure point group operations)
out of a pool of 4 trial point group operations.


The static configuration has the point symmetry S_2 .

Analysis of symmetry for dynamics (positions and initial velocities):

Subroutine DYNSYM returns: Found 2 space group operations
(whereof 2 operations were pure point group operations)
out of a pool of 2 trial space group operations
(whereof 2 operations were pure point group operations)
and found also 2 'primitive' translations


The dynamic configuration has the point symmetry S_2 .
=====================================

So, is my structure monoclinic or triclinic? Why are two structures listed?
Thanks.

The unit cell is monoclinic. VASP also checks if the unit cell can be obtained by smaller primitive cell using the subroutine PRICEL. For your case, VASP found such a primitive cell, but it corresponds to a triclinic structure.

Thank you for the reply.