  In electronic structure calculations, various material properties can be
obtained by means of computing the total energy of a system as well as
derivatives of the total energy w.r.t. atomic positions. The derivatives, also
known as Hellman-Feynman forces, require, because of practical computational
reasons, the discretized charge density and wave functions having continuous
second derivatives in the whole solution domain. We describe an application of
isogeometric analysis (IGA), a spline modification of finite element method
(FEM), to achieve the required continuity. The novelty of our approach is in
employing the technique of B\'ezier extraction to add the IGA capabilities to
our FEM based code for ab-initio calculations of electronic states of
non-periodic systems within the density-functional framework, built upon the
open source finite element package SfePy. We compare FEM and IGA in benchmark
problems and several numerical results are presented.
