Energetics of the Light Actinides in a Full Charge Density Scheme

Abstract

During the last years many attempts have been made to explain the unusual equilibrium bulk properties of the light actinides using ab initio methods for the calculation of the total energy [1, 2, 3]. With the subsequent filling of the f-conduction band, for Np and especially for Pu, the observed equilibrium volumes are reasonably larger and the bulk moduli are smaller than the calculated ones; in contrast to the theoretical predictions the measured equilibrium volume for plutonium is larger than that of neptunium. According to the recent study by Wills and Eriksson [3] this discrepancy might be ascribed to the fact that in the calculations not the observed structures were used. To examine this possibility, in this paper we carried out a recently developed [4, 5] full charge density LMTO-ASA (linear muffin-tin orbital - atomic sphere approximation) calculation for these elements using the observed a-structures. We will show that our calculated values are fairly accurate for Th, Pa and U, but the anomalously large plutonium volume is not reproduced and our results show similar monotonic decrease in the atomic volumes as the earlier calculations. Therefore we conclude that this discrepancy reflects the limitations of the local density scheme, namely that the quasi-localized, relativistic f-conduction band is improperly treated in this approximation.