The use of TAU in calculation VASPsol

[anwin-john]

Initially, I used default TAU value of, TAU= 0.000525 for the calculation of *COH system adsorbed on Cu(111) surface. Then, I used TAU = 0. In the USAGE.md file it is written,
You can also change the surface tension parameter tau in the INCAR file. For example, to neglect the the cavitation energy contribution, set TAU = 0 in the INCAR file. Note that you may have to increase the cutoff energy to converge the caviation energy because the grid must be fine enough to resolve the cavity surface.
I got the results as,
TAU= 0.000525 energy(sigma->0) = -270.868292
TAU= 0.000000 energy(sigma->0) = -271.009225
There is a difference of 0.14 eV.

These are my input parameters

Parameters related to accuracy of the simulation
PREC= Normal # Precision of the calculation
ALGO= Normal # Selects the block Davidson diagonalization algorithm
LREAL= .FALSE. # Evaluation of projection operators in reciprocal space
ENCUT= 450 # Plane wave cutoff in eV
EDIFF= 1E-5 # Converge DFT energy till variation is less than 1E-5 eV
NELMIN= 5 # Minimum number of times to run SCF loop, irrespective of EDIFF convergence criterion
ISTART=1
ICHARG=1
ISYM=0

Accelerating convergence through electronic smearing
ISMEAR= 1 # Gaussian smearing to accelerate convergence
SIGMA= 0.1 # Width of Gaussian for smearing electronic distribution

Spin polarization setting
ISPIN= 2 # Spin-polarized calculation, i.e., taking spin into account
IDIPOL=3 #For Cu ispin is not needed as Cu is not magnetic
LDIPOL=.TRUE.

Output settings
LCHARG= .TRUE. # Do not write CHGCAR
LWAVE= .TRUE. # Do not write WAVECAR

Parallelization options
NPAR= 8 # Number of bands that are treated in parallel; NPAR ~ sqrt(number of cores)

Exchange-correlation functional settings
GGA= PE # Chooses PBE XC functional
IVDW= 12 # Adds dispersion to DFT using Grimme's D3 method, with Becke-Johnson (BJ) damping, see: 10.1021/jp501237c

Cell optimization details Not needed while finding the converged settings
IBRION= 2 # Optimize ion positions
ISIF= 2 # Optimize simulation cell, i.e., vary cell volume, cell shape and ion positions
EDIFFG= -0.01 # Stop optimization if forces on all atoms are less than 0.01 eV/A
NSW= 500 # Number of optimization steps to carry out

Solvation Parameters
LSOL = .TRUE. #implicit Solvation
LAMBDA_D_K = 3.04 #Debye Length in A

As per the paper Implicit self-consistent electrolyte model in plane-wave density-functional theory I quote "to obtain accurate values for the cavitation energy requires a high plane-wave basis set cutoff energy of 1000 eV."

Question:
Does that mean my calculation with ENCUT of 450 eV led to a wrong calculation of cavitation energy? Even if the cavitation energy calculation is wrong, how would it lead to such a large change in energy just due to cavitation?

[ericfonseca95]

Hello,

Using small values of ENCUT when assessing the cavitation energy can lead to errors. This is due to bad resolution of the surface where VASPsol looks at the gradient of the shape function. The poor resolution can cause fluctuations in energy and convergence issues. This is why the recommended ENCUT is around 1000 eV.

Eric

[andyscratch]

The new nonlinear model, VASPSol++ circumvents this problem. Come in very handy