M ar 1 99 8 LAPW vs . LMTO full - potential simulations and anharmonic dynamics of KNbO 3

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 “NFP” implementation of the full-potential linear muffin-tin orbital (FP-LMTO) method by M. Methfessel and M. van Schilfgaarde, we perform a comparative study of the ferroelectric instability in KNbO3 by FP-LMTO and full-potential linear augmented planewave (LAPW) method. 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 Schrödinger equation in the model of noninteracting anharmonic oscillators via the expansion in hyperspherical harmonics. Preliminary results are given for the t1u vibrational modes in cubic KNbO3.