  With the aim to get an insight in the origin of differences in the earlier
reported calculation results for KNbO3 and to test the recently proposed
implementation of the FP-LMTO method by Methfessel and van Schilfgaarde, we
perform a comparative study of the ferroelectric instability in KNbO3 by
FP-LMTO and LAPW methods. It is shown that a high precision in the description
of the charge density variations over the interstitial region in perovskite
materials is essential; the technical limitations of the accuracy of
charge-density description apparently accounted for previously reported slight
disagreement with the LAPW results. With more accurate description of the
charge density by sufficiently fine real-space grid, the results obtained by
both methods became almost identical.
  In order to extract additional information (beyond the harmonic
approximation) from the total energy fit obtainable in total-energy
calculations, a scheme is proposed to solve the multidimensional vibrational
Schroedinger equation in the model of non-interacting anharmonic oscillators
via the expansion in hyperspherical harmonics.
