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I am trying to use the lobster program to calculate COHP, according to the explanation in the literature, it is better to add unoccupied atom orbitals into the basis set.

The initial step in projection is finding a suitable choice of local auxiliary basis functions. For reasons of simple chemical interpretation, LOBSTER employs minimal basis sets that nonetheless carry the correct nodal behavior in the core region, which is necessary to fit PAW wavefunctions. LOBSTER first came with contracted primitive Slater-type orbitals (STOs) fitted to atomic functions,14 a reasonable choice for post-processing bonding information. There are also systems, however, where the bonding situation requires additional basis functions which deviate from those of the corresponding free atoms. In elemental beryllium, for example, its 2p levels are unoccupied in the free atom. For bulk Be, however, the 2p levels do get involved into bonding and define the metallic character, so the Be basis set must also include 2p functions.

For example, here is an input file for lobster to analyze COHP between Ni and Fe elements, whose valence electrons are in 3d and 4s orbitals, but the input file also uses 4p orbitals.

COHPstartEnergy  -10
COHPendEnergy     6
usebasisset pbeVaspFit2015
basisfunctions Fe 3d 4s 4p
basisfunctions Ni 3d 4s 4p

cohpbetween atom 1 and atom 2

My question is:

  1. Could I also use 4d, 4f etc. for elements like Ni and Fe?
  2. How should I decide how many unoccupied atom orbitals I should add into the basis set?
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  • $\begingroup$ Please limit your post to one specific question. If you have any lingering doubts after receiving an answer, you ask an additional question in a separate post. $\endgroup$
    – Tyberius
    Commented Jun 1, 2022 at 16:21

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