I would like to know the definition of altra soft pseudopotential.(in solid state
An ultra soft pseudopotential is defined by the fact that you need a low cut off value for the plane waves. This means that you need a lot of plane waves to properly approximate the (valence) electron-nucleus potential. In other words, the pseudopotential yields a good approximation for the e-n-potential (and ofcourse the all electron-wavefunction) starting from a small distance away from the core electrons. The plane wave cut off is defined as the distance between the core electrons and the valence electrons of an atom.
More to be found here and here
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