[QE-users] tutorial for the FCP method

[Otani Minoru]

Dear Lucian,

I made those examples in PW/examples/ESM_example directory. Here, I am going to explain the purpose of each examples.

1) ESM
One can understand the overall usage of the effective screening medium (ESM) method. More precisely, when you plot the esm1 file, you can see the difference of the calculated electrostatic potentials depending on the boundary conditions, i.e., pbc, bc1, bc2, and bc3. By changing the parameter "tot_charge", you can see how the Fermi energy changes in a Constant-N calculation: Constant-N means that we fix the total number of electron (N) during the SCF calculation. If you want to understand more detail about the method and configuration of the slab model, please read the original paper:
Physical Review B 73, 115407 (2006)

2) FCP & 3) GCSCF
Unlike the Constant-N calculation, the second (FCP) and the third (GCSCF) examples, we can conduct an electronic structure calculation with fixed chemical potential for electron (constant-mu). FCP stands for fictitious charge particle and the original paper is the following:
Physical Review Letters 109, 266101 (2012).
GCSCF is the abbreviation for the grand canonical SCF and the original paper is below:
J. Chem. Phys. 146, 114104 (2017).

You can find the difference between FCP and GCSCF calculations in the following paper:
Physical Review Materials 5, 065001 (2021).
As stated in the paper, FCP and GCSCF have their advantages in the constant-mu calculations. Thus we have implemented both methods for users convenience. 

Best regards,
Minoru

--------------------------------------------------------------------------------------------------------
Minoru Otani, Ph.D., Professor
Center for Computational Sciences and Faculty of Pure and Applied Sciences, 
University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan
TEL: +81-29-853-4273
FAX: +81-29-853-6406
E-mail: otani at ccs.tsukuba.ac.jp
Web: https://www2.ccs.tsukuba.ac.jp/public/otani/
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> On Apr 11, 2022, at 15:48, Lucian D. Filip <lucian.filip at infim.ro> wrote:
> 
> Hello all,
> 
> Is there a tutorial out there that explains in detail how to use the FCP method? I have read the one in the PW/examples on github but I can't figure out what it is trying to accomplish.
> 
> It is split into three types of calculations: 1) ESM calculation; 2) FCP calculation; 3) GCSCP calculation. Why is that? Are they related? Is this the order one is supposed to use to obtain a what exactly?
> 
> Can someone explain the purpose of each steps taken in this example?
> 
> 
> Thank you!
> 
> 
> Lucian
> 
> -- 
> Dr. Lucian Dragos Filip
> National Institute of Materials Physics
> Atomistilor str. 405A
> PO Box MG. 7
> Magurele, 077125
> Bucharest, Romania
> E-mail:   lucian.filip at infim.ro
> Website: http://lucianfilip.wordpress.com
> 
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