Ultrasoft pseudopotentials USPPs

Most plane wave calculations use ultrasoft pseudopotentials (USPP),15,28 which describe the core electrons of atoms in a mathematically efficient form that greatly reduces the computational cost associated with heavy atoms. An increasing number of calculations used the projector augmented wave (PAW) approach instead.28 In most circumstances where both approaches can be used, the differences between USPP and PAW calculations are minor. Some exceptions to this observation include transition metals with large magnetic moments (e.g., Fe) and alkali metals.28... 

In this paper we report on the stability of free-standing single-crystal SiNWs with respect to their morphology (involving different surfaces and surface reconstructions), orientation (namely nanowires with <001>, <011>, <111>, and <112> axes), and diameter (from 2 to 5 nm) by applying the ab initio ultrasoft pseudopotential (USPP) method. 

NCPP, norm-conserving pseudopotentials USPP, ultrasoft pseudopotentials PBE-TS denotes DFT-PBE computations with dispersion correction of Tkatchenko and Scheffler applied. 

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