USPEX 9.4.4 manual: INCAR_* files in Specific/ folder for VASP

3.4 `INCAR_*` files in `Specific/` folder for VASP

To run USPEX correctly, there are some hints on the files in Specific/ folder to control the structure relaxation in USPEX. We take example of VASP as an external code:

Your final structures have to be well relaxed, and energies — precise. The point is that your energy ranking has to be correct (to check this, look at E_series.pdf file in the output).
Your POTCAR files: To yield correct results, the cores of your pseudopotentials (or PAW potentials) should not overlap by more than 10–15%.
To have accurate relaxation at low cost, use the multistage relaxation with at least three stages of relaxation for each structure, i.e. at least three INCAR files (INCAR_1, INCAR_2, INCAR_3, …). We usually set 4–5 stages of relaxation.
Your initial structures will be usually very far from local minima, in such cases it helps to relax atoms and cell shape at constant volume first (ISIF=4 in INCAR_1,2), then do full relaxation (ISIF=3 in INCAR_3,4), and finish with a very accurate single-point calculation (ISIF=2 and NSW=0 in INCAR_5).
Exceptions: when you do fixed-cell predictions, and also in evolutionary metadynamics (except full relaxation) you must have ISIF=2.
When your volume does not change, you can use default plane wave cutoff. When you use ISIF=3, you must increase it by 30–40%, otherwise you get a large Pulay stress. Also your convergence criteria can be loose in the beginning, but have to be tight in the end: e.g., EDIFF=1e-2 and EDIFFG=1e-1 in INCAR_1, gradually tightening to EDIFF=1e-4 and EDIFFG=1e-3 in INCAR_4. The maximum number of timesteps (NSW) should be sufficiently large to enable good relaxation, but not too large to avoid wasting computer time on poor configurations. The larger your system, the larger NSW should be.
Choosing an efficient relaxation algorithm can save a lot of time. In VASP, we recommend to start relaxation with conjugate gradients (IBRION=2 and POTIM=0.02) and when the structure is closer to local minimum, switch to IBRION=1 and POTIM=0.3.
Even if you study an insulating system, many configurations that you will sample are going to be metallic, so to have well converged results, you must use “metallic” treatment — which works both for metals and insulators. We recommend the Methfessel-Paxton smearing scheme (ISMEAR=1). For a clearly metallic system, use ISMEAR=1 and SIGMA=0.1–0.2. For a clearly insulating system, we recommend ISMEAR=1 and SIGMA starting at 0.1 (INCAR_1) and decreasing to 0.03–0.04.

Here we provide an example of INCAR files for carbon with 16 atoms in the unit cell, with default ENCUT=400 eV in POTCAR:

INCAR_1:
    PREC=LOW
    EDIFF=1e-2
    EDIFFG=1e-1
    NSW=65
    ISIF=4
    IBRION=2
    POTIM=0.02
    ISMEAR=1 
    SIGMA=0.10

INCAR_2:
    PREC=NORMAL
    EDIFF=1e-3
    EDIFFG=1e-2
    NSW=55
    ISIF=4
    IBRION=1
    POTIM=0.30
    ISMEAR=1 
    SIGMA=0.08

INCAR_3:
    PREC=NORMAL
    EDIFF=1e-3
    EDIFFG=1e-2
    ENCUT=520.0
    NSW=65
    ISIF=3
    IBRION=2
    POTIM=0.02
    ISMEAR=1 
    SIGMA=0.07

INCAR_4:
    PREC=NORMAL
    EDIFF=1e-4
    EDIFFG=1e-3
    ENCUT=600.0
    NSW=55
    ISIF=3
    IBRION=1
    POTIM=0.30
    ISMEAR=1 
    SIGMA=0.06

INCAR_5:
    PREC=NORMAL
    EDIFF=1e-4
    EDIFFG=1e-3
    ENCUT=600.0
    NSW=0
    ISIF=2
    IBRION=2
    POTIM=0.02
    ISMEAR=1 
    SIGMA=0.05

The philosophy of METADYNAMICS is very similar to USPEX, except that we DO NOT change the cell shape during the META evolution. Therefore, we need to put ISIF=2 for all META steps. If the full relaxation mode is on, we can put ISIF=3 for the steps of full relaxation. Therefore, if we have the following set up:

% abinitioCode 
1 1 1 (1 1) 
% ENDabinit

the ISIF should be “2 2 2 3 3” for INCAR_1, …, INCAR_5 correspondingly.

Different from USPEX, VC-NEB method doesn’t need a structure relaxation from the external codes, which runs the structure relaxation in VC-NEB itself with the forces from external code calculation. Thus, there are some differences in the files. Take VASP INCAR files for example, we need to set NSW=0 to avoid the structure relaxation, but with ISIF=2 or 3 to extract the forces on the atoms, and the stress tensor on the lattice in VASP. We also suggest to use PREC=Accurate to have a good estimation for the forces and stress to accelerate the calculations for VC-NEB. An example of INCAR file for VC-NEB is presented below:

INCAR_1:
    PREC=Accurate
    EDIFF=1e-4
    EDIFFG=1e-3
    ENCUT=600.0
    NSW=0
    ISIF=2
    IBRION=2
    POTIM=0.02
    ISMEAR=1 
    SIGMA=0.05

3.4 INCAR_* files in Specific/ folder for VASP

3.4 `INCAR_*` files in `Specific/` folder for VASP