[Pw_forum] Electron affinity calculation using Quantum Espresso

Andrea Ferretti andrea.ferretti at unimore.it
Sun Jan 31 13:15:11 CET 2016

Dear Kanak,

> "However, a different definition holds for finite systems. In this case, the electron affinity is defined as
> EA=E(N)-E(N+1) where E(N) and E(N+1) are the total ground-state energies in the neutral (N) and single charged (N+1)
> configurations"

this is the so-called Delta-scf method, applied to the calculation of 
electron affinities (similarly one can compute IP=E(N-1)-E(N) )

Concerning the EA of MoS2, there are two problems

(1) delta-scf methods work only for finite systems (when using standard
     DFT functionals such as LDA or GGA's), while MoS2 is extended.

(2) even for finite systems, EA's may be difficult to compute since
     approximate DFT functionals may not be able to bind the extra electron
     to compute E(N+1) (this ia actually the most common situation,
     especially when dealing with a very flexible basis set as plane
     waves). This is actually not specific to MoS2, but a general problem.

     In these cases, a more effective way is to compute the N+1 eigenvalue
     when 0.5 electrons have been added to the system (Slater 1/2 type of
     approach), approximating by taylor expansion the E(N)-E(N+1)
     difference without requiring to bind a full electron (just half of it)

Feasible alternatives to compute HOMO and LUMO bands (IP and EA) for
extended systems such as MoS2 are based on GW calculations...

Take care

> If I want to calculate affinity for MoS2 monolayer, according to this reference, I need to calculate ground state energy
> for neutral system and then charged system and then find out the difference between the two. Is this sufficient? Please
> let me know.
> Thanks in advance.
> Sincerely yours
> Kanak Datta
> Dept. of EEE, BUET

Andrea Ferretti, PhD
S3 Center, Istituto Nanoscienze, CNR
via Campi 213/A, 41125, Modena, Italy
Tel: +39 059 2055322;  Skype: andrea_ferretti
URL: http://www.nano.cnr.it

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