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what's wrong with the original interface?

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Chengcheng Xiao edited this page Jul 4, 2020 · 1 revision

Original Symptoms

In VASP (version 5.4.4), the VASP2WANNIER90v2 compiler flag was added as a interface to the WANNIER90 (version 2.X+) program. However, with spin-orbital coupling turned on, this interface cannot correctly calculate the number of projections needed.

The symptom is quite obvious, wrong number of projections are done in the projection routine, causing VASP to either freeze or crash.

For example:

Without spinor tag

The projection block in .win file

begin projections
site1: s,p
end projections

VASP will do the following

site 1 projection s  (spin_1)
site 1 projection s  (spin_2)
site 1 projection px (spin_1)
site 1 projection px (spin_2)
site 1 projection py (spin_1)
site 1 projection py (spin_2)
...
...
site 1 projection s  (spin_1) [The program got stuck here, because the num_proj has already been reached]
site 1 projection s  (spin_2)
site 1 projection px (spin_1)
site 1 projection px (spin_2)
site 1 projection py (spin_1)
site 1 projection py (spin_2)
...

with spinor projection

The projection block in .win file

begin projections
site1: s
site1: pz(d)
end projections

VASP will do the following:

(VASP SPINORS=1)
site 1 projection s  (WAN spin_1)
site 1 projection s  (WAN spin_2)
site 1 projection pz (WAN spin_2)
...
...
(VASP SPINORS=2)
site 1 projection s  (WAN spin_1) [The program got stuck here, because the num_proj has already been reached]
site 1 projection s  (WAN spin_2)
site 1 projection pz (WAN spin_2)
...

Why?

In WANNIER90 v1.2, in the parameters.F90 file, there is:

if(spinors) num_proj=num_wann/2

This means they treated the two spin channel the same way. Both channels will use the same projection functions.

However, in WANNIER90 v2.1+, changes were made in the parameters.F90, they added a spin counter spn_counter:

VALUE MEANING
1 only spin_up or only spin_down
2 both spin_up and spin_down

Each projection's spin channel were then determined by:

if (spinors) then
  if (spn_counter == 1) then
    if (proj_u_tmp) input_proj_s(counter) = 1
    if (proj_d_tmp) input_proj_s(counter) = -1
  else
    if (loop_s == 1) input_proj_s(counter) = 1
    if (loop_s == 2) input_proj_s(counter) = -1
  endif
  input_proj_s_qaxis(:, counter) = proj_s_qaxis_tmp

this means the spin channels are treated separately. If the wavefunctions are spinors, each projection's spin channels are labled by proj_s tag.

In the VASP2WANNIER90 interface, the projection scheme loops over the spin channels and projection sites by:

spinor: DO ISPINOR=1,WDES%NRSPINORS
sites : DO ICNTR=1,num_bands_tot

Within the loop, only proj_l proj_m proj_radial are used.

When compiled against the WANNIER90 v2.0+ version, the spin channel information is already included in proj_s. If proj_s is ignored, the total number of iterations are corrupted.

Fix

Utilize proj_s as the spin channel indicator, project corresponding Bloch states on to the orbital functions with correct spin channel.

If the spin quantization axis is set, full spinor guiding functions will be constructed, the rotation of those spinor wavefunctions are done by dotting with the eigenvectors of the Pauli matrix. PAW projector coefficients' contribution to the overlap will also be calculated for said rotations.

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