Whatever the system considered and the code used, one needs to provide more inputs than just the atomic positions and types. Most codes suggest some values for these inputs. However, their tuning may still be necessary as default values may very well be suited for some supposedly standard situations and irrele­vant for others. Blind use of ab initio codes may thus lead to disappointing errors. Indeed, not all these choices are trivial, so mistakes can be hard to notice for the beginner. Choices are usually made out of experience, after considering some test cases needing small calculation time.

One can distinguish between choices that should be made only once at the beginning of a study and calculation parameters that can be tuned calculation by calculation. The main unchangeable choices are the exchange and correlation functional and the pseudopotentials or PAW atomic data for the various atomic types in the calculation.

First, one has to choose the flavor of the exchange and correlation functional that will be used to describe the electronic interactions. Most of the time one chooses either an LDA or a GGA func­tional. Trends are known about the behavior of these functionals: LDA calculations tend to overestimate the bonding and underestimate the bond length in bulk materials, the opposite for GGA. However, things can become tricky when one deals with defects as energy differences (between defect-containing and defect-free cells) are involved. For insulating materi­als or materials with correlated electrons, the choice of the exchange and correlation functional is even more difficult (see Section 1.08.5).

The second and more definitive choice is the one of the pseudopotential. We do not mean here the choice of the pseudoization scheme but the choice of the pseudopotential itself. Indeed, calculated ener­gies vary greatly with the chosen pseudopotential, so energy differences that are thermodynamically or kinetically relevant are meaningless if the various calculations are performed with different pseudopo­tentials. The determination ofthe shape ofthe atomic basis set in the case of localized bases is also of importance, and it is close in spirit to the choice of the pseudopotential except that much less basis sets than pseudopotentials are available.

More technical inputs include

• the k-point sampling. The larger the number of k points to sample the Brillouin zone, the more accurate the results but the heavier the calcula­tions will be. This is especially true for metallic systems that need fine sampling of the Brillouin zone, but convergence with respect to the number of k points can be accelerated by the introduction of a smearing of the occupations of electronic levels close to the Fermi energy. The shape and width of this smearing function is then an addi­tional parameter.19

• the number ofplane waves (obviously for plane wave codes but also for some other codes that also use FFT). Once again the larger the number of plane waves, the more accurate and heavier the calculation.

• the convergence criteria. The two major conver­gence criteria are the one for the self-consistent loop of the calculation of the ground-state electronic wave functions and the one to signal the convergence of a relaxation calculation (with some threshold depending on the forces acting on the atoms).