As explained above, pseudoization schemes are espe­cially relevant for plane wave codes. All pseudoiza — tion schemes are obtained by calculations on isolated atoms or ions. The real potential experienced by the valence electrons is replaced by a pseudopotential coming from mathematical manipulations. A good pseudopotential should have two apparently contra­dictory qualities. First, it should be soft, meaning that the wave function oscillations should be smoothened as much as possible. For a plane wave basis set, this means that the number of plane waves needed to represent the wave functions is kept minimal. Second, it should be transferable, which means that it should correctly represent the real interactions of valence electrons with the core in any kind of chemical envi­ronment, that is, in any kind of bonding (metallic, covalent, ionic), with all possible ionic charges or covalent configurations conceivable for the element under consideration. The generation of pseudopo­tentials is a rather complicated task, but nowadays libraries of pseudopotentials exist and pseudopoten­tials are freely available for almost any element, though not with all the pseudoization schemes.

One can basically distinguish norm-conserving pseudopotentials, ultrasoft pseudopotentials, and PAW formalism. Norm-conserving pseudopotentials were the first ones designed for ab initio calculations.12 They involve the replacement of the real valence wave function by a smooth wave function of equal norm, hence their name. Such pseudopotentials are rather easy to generate, and several libraries exist with all elements of the periodic table. They are rea­sonably accurate although they are still rather hard, and so they are less and less used in plane wave codes but are still used with atomic-like basis sets. Ultrasoft pseudopotentials13 remove the constraint of norm equality between the real and pseudowave functions. They are thus much softer though less easy to gen­erate than norm-conserving ones. The Projector Augmented Wave14 formalism is a complex pseudoi­zation scheme close in spirit to the ultrasoft scheme but it allows the reconstruction of the real electronic den­sity and the real wave functions with all their oscilla­tions, and for this reason this method can be considered an all-electron method. When correctly generated, PAW atomic data are very soft and quite transferable. Libraries of ultrasoft pseudopotentials or PAW atomic data exist, but they are generally either incomplete or not freely available.

Plane wave codes in use in the nuclear materials community include VASP15 with ultrasoft pseudopo­tentials and PAW formalism, Quantum-Espresso16 with norm-conserving and ultrasoft pseudopotentials and PAW formalism, and ABINIT17 with norm — conserving pseudopotentials and PAW formalism.

Note that for a specific pseudoization scheme many different pseudopotentials can exist for a given element. Even if they were built using the same valence orbitals, pseudopotentials can differ by many numerical choices (e. g., the various matching radii) that enter the pseudoization process.

We present in the following a series of practical choices to be made when one wants to perform ab initio calculations. But the first and certainly most important of these choices is that of the ab initio code itself as different codes have different speeds, accuracies, numerical methods, features, input files, and so on, and so it proves quite difficult to change codes in the middle of a study. Furthermore, one observes that most people are reluctant to change their usual code as the investment required to fully master the use of a code is far from negligible (not to mention the one to master what is in the code).