Monolayer molybdenum disulfied

This tutorial explains how to optimize the cell parameter of a system with internal degree(s) of freedom, by taking monolayer molybdenum disulfied (MoS2) as an example.

An example input file for monolayer MoS2is as follows:

#
# Monolayer MoS2
#
TASK      GEO_OPT
WF_OPT    DAV
NTYP      2
NATM      3
TYPE      0
GMAX      6.0
GMAXP     20.0
MIX_ALPHA 0.3
SMEARING  MP
WIDTH     0.0010
NEG       28
GEO_OPT   QMD
DTIO      200.0
KPOINT_MESH    12 12  1
KPOINT_SHIFT    F  F  F
CELL  6.04 6.04 18.8972612463  90.00000000  90.00000000 120.00000000
&ATOMIC_SPECIES
 Mo  95.95 pot.Mo_pbe1
 S   32.06 pot.S_pbe1
&END
&ATOMIC_COORDINATES CRYSTAL
      0.333333333333      0.666666666667      0.000000000000    1    1    1
      0.666666666667      0.333333333333      0.160000000000    1    1    2
      0.666666666667      0.333333333333     -0.160000000000    1    1    2
&END

Note

It is not necessary to use the MP smearing as MoS2. Use WIDTH without SMEARING, if there is not convergence issue (this can be the case when the band gap is very small)

For each lattice constant, we optimize the internal degree of freedom using the quenched molecular dynamics (GEO_OPT QMD). We repeat the structural optimization for a given set of lattice constants (from 6.0 Bohr to 6.1 Bohr with a step width of 0.01 Bohr), and obtain the total energy as a function of the lattice constant as follows:

../_images/etot_mos2_fit.png

By fitting the total energy to the 6-th order polynomial, we obtain the equilibrium lattice constant of 6.036 Bohr (3.194 Angstrom), which is comparable to that reported in a literature (see for example, 3.19 Angstrom with PBE, Phys. Rev. B 89, 121103 (2014)).