[QE-users] Vacuum-slab-vacuum (open boundary conditions)

[Giuseppe Mattioli]

Dear user (please always sign the posts with name and affiliation)
Reading the manual and the papers (with a bit of testing) should help  
answer your questions

                    'esm' :
                         Effective Screening Medium Method.
                         For polarized or charged slab calculation, embeds
                         the simulation cell within an effective semi-
                         infinite medium in the perpendicular direction
                         (along z). Embedding regions can be vacuum or
                         semi-infinite metal electrodes (use "esm_bc" to
                         choose boundary conditions). If between two
                         electrodes, an optional electric field
                         ('esm_efield') may be applied. Method described in
                         M. Otani and O. Sugino, "First-principles calculations
                         of charged surfaces and interfaces: A plane-wave
                         nonrepeated slab approach", PRB 73, 115407 (2006).

                         NB:
                            - Two dimensional (xy plane) average charge density
                              and electrostatic potentials are printed out to
                              'prefix.esm1'.

                            - Requires cell with a_3 lattice vector along z,
                              normal to the xy plane, with the slab centered
                              around z=0.

                            - For bc2 with an electric field and bc3 boundary
                              conditions, the inversion symmetry along  
z-direction
                              is automatically eliminated.

                            - In case of calculation='vc-relax', use
                              cell_dofree='2Dxy' or other parameters so that
                              c-vector along z-axis should not be moved.

                         See "esm_bc", "esm_efield", "esm_w", "esm_nfit".

    +--------------------------------------------------------------------

    +--------------------------------------------------------------------
    Variable:       esm_bc

    Type:           CHARACTER
    See:            assume_isolated
    Default:        'pbc'
    Description:
                    If "assume_isolated" = 'esm', determines the boundary
                    conditions used for either side of the slab.

                    Currently available choices:

                    'pbc' :
                         (default): regular periodic calculation (no ESM).

                    'bc1' :
                         Vacuum-slab-vacuum (open boundary conditions).

                    'bc2' :
                         Metal-slab-metal (dual electrode configuration).
                         See also "esm_efield".

                    'bc3' :
                         Vacuum-slab-metal

> What does this mean "open boundary conditions", it is not clear!

It means that the asymptotic values of the potential along +z and -z  
are not anymore constrained by periodic boundary conditions. The  
behavior of the potential depends on the esm_bc value. using bc2  
(metal-slab-vacuum) you should in principle (bc1 works very well in my  
experience; I've never personally tested bc2) be able to change the  
Fermi level within the slab to simulate source/drain electrodes in  
contact with organic semiconductors.

HTH
Giuseppe

Quoting Ola Starkolis <qmpotential at gmail.com>:

> What does this mean "open boundary conditions", it is not clear!
> I would like to run metal/organic/vacuum and using
>
>  assume_isolated = "esm"
>       esm_bc          = "bc1"
>
> I suppose that the system is periodic in x and y directions and
> non-periodic in z. Is it right?
>
> Thank you so much for your help in advance.



GIUSEPPE MATTIOLI
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
Via Salaria Km 29,300 - C.P. 10
I-00015 - Monterotondo Scalo (RM)
Mob (*preferred*) +39 373 7305625
Tel + 39 06 90672342 - Fax +39 06 90672316
E-mail: <giuseppe.mattioli at ism.cnr.it>