I am looking for suggestions on how to approach DFT calculations for larger systems (48-150 atoms/unit cell) and if any additional considerations are needed when dealing with organics.
The overall aim is to model the energy-loss function of the materials, but first, a ground state calculation is of course needed.
The simplest system I am working with has 48 atoms per unit cell (4 species) and is a form of Hydroxylapatite ( structure was found on the Crystallography Open Database and converted into an xml input via CIF2Cell. COD ID: 1010647 )
The closest I am getting is with a 1x1x1 k-mesh. In 18 hours it has completed 3 iterations and the data suggests it is in fact converging, but may take several (maybe more than 20) more iterations to actually satisfy the default convergence criteria.
The GSC info is here:
<groundstate swidth="0.01" nempty="10" xctype="GGAPBEsol" ngridk="1 1 1" lradstep="4" gmaxvr="14.0" lmaxvr="8" rgkmax="7.0" maxscl="30"/>
I was considering increasing nempty to something closer to 50 or so since the number of valence states is 155.
I did get a warning for SCL1 and SCL2
Warning(occupy): smallest valence eigenvalue less thanminimum linearization energy : -5.164780185 -1.645800000
for s.c. loop 1
Warning(occupy): smallest valence eigenvalue less thanminimum linearization energy : -3.935421531 -1.645800000
for s.c. loop 2
Since loop 4 is in progress, that suggests loop 3 did not meet this condition.
I hope I have provided adequate information here.