[QE-users] [EXT] Questions about work function of metals in water(implicit water)

[Andreussi, Oliviero]

Dear Bingxin,

I am glad you are considering using Environ for your work. You may want to join the google group of Environ users, where some of the common issues and solutions are discussed, you can find it in the following: https://groups.google.com/g/quantum-environ-users

Concerning your questions, I should start with one important point: Environ is NOT coupled with the PP code, so PP will not know about the Environ contributions to the electrostatic potential (but you can still use it to plot the electronic density or other properties that do not require Environ modifications).

In your setup of the Environ calculation you include a parabolic correction for the electrostatic potential. This will ensure that the potential in the environment region will go to zero, so that the Fermi energy reported in the output can be used to estimate the ‘work function’, or more appropriately the potential of zero charge. The only thing to take into account is a constant shift to the potential which is due to the way ionic nuclei are handled in Environ (gaussians instead of point charges). This shift is reported in the output of the pw calculation and should be added to the Fermi energy to get the correct PZC.

I think this should fix the main issue. If you still want to plot the electrostatic potential in the cell in the presence of the environment, you can increase the verbosity of the Environ output (verbosity = 2 or higher). This will generate .cube files that contain the main physical quantities defined in the real space grid. You can visualize .cube files with several visualization packages and they are reasonably straightforward to postprocess.

Best,

Oliviero

Oliviero Andreussi
--
Assistant Professor
Department of Physics
University of North Texas
Email: oliviero.andreussi at unt.edu<mailto:oliviero.andreussi at unt.edu>
Phone: +1-(940)-369-5316
Skype: olivieroandreussi
Web: https://www.materialab.org

On Feb 23, 2021, at 7:16 PM, Li, Bingxin <bingxin.li18 at imperial.ac.uk<mailto:bingxin.li18 at imperial.ac.uk>> wrote:

Dear all,

Recently I am using Environ + QE to calculate the work function of Mg(0001) surface in different environment (vacuum/implicit water).  Firstly I was doing a geometry optimisation calculation of Mg(0001) slab without environment packages. In this case, the fermi energy of the slab is -0.2249 eV and the vacuum potential is around 3.4713 eV(obtained using the pp.x and avg.x). Therefore, the work function can be simply calculated using 3.4713 – (-0.2249) = 3.6962 eV (which is close to the reference value: 3.62 eV).

Then I use the environment packages and turn on the implicit model, the input file for the environment part is as follows:

&ENVIRON
   !
   verbose = 0
   environ_thr = 1.D0
   environ_type = 'input'
   env_static_permittivity = 78.3(water)/1(vacuum)
   env_surface_tension = 0.D0
   env_pressure = 0.D0
   env_electrostatic = .true.
   !
/
&BOUNDARY
   !
   solvent_mode = 'full'
   !
/
&ELECTROSTATIC
   !
   pbc_correction = 'parabolic'
   pbc_dim = 2
   pbc_axis = 3
   tol = 5.D-13
   !
/

The calculations for the slab in vacuum/water were done without any errors. However, in the vacuum case, the fermi energy (considering the parabolic correction) of the slab is -3.2031 + 0.4671 = -2.736 eV. The vacuum (enviroment) potential is 3.105 eV (using pp.x and avg.x). The work function is vacuum with the environment package is 3.105 -  (-2.736) = 5.841 eV, which is not correct. I guess this error answer is due to the wrong vacuum potential. That is to say, the environment potential in this case should not calculate in the normal way (pp.x + avg.x) or the environment potential should be corrected in some way. However, I have no idea about how to obtain the correct environment(vacuum/water) potential when the environment packages are used. I was wondering that could anyone who knows how to calculate the work function of the metal slab in the implicit environment correctly please help me on this issue?

I am looking forward to your reply.

Best Regards,
Bingxin

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