We present first-principles calculations of the equilibrium volumes and crystal structures of the light actinides (ThPu). The calculated equilibrium volumes for f.c.c. Th, b.c.t. Pu, α-U, and β-Np are found to agree reasonably well with the experimental data, and when comparing the total energies of the b.c.c., f.c.c., b.c.t., α-U, and β-Np structures we obtain the correct crystal structures for all studied systems. Also, the calculated equilibrium volumes for ThPu, using a hypothetical f.c.c. structure, have been calculated and it is demonstrated that although spin—orbit coupling is included in these calculations the calculated equilibrium volume of Pu is smaller than for Np, in disagreement with experiment. Moreover, the calculated tetragonal elastic constant, C′, is shown to be negative for b.c.c. U, b.c.c. Np, b.c.c. Pu and f.c.c. Pu. Thus, our zero temperature calculations suggest that the b.c.c. structure is unstable for these elements and that f.c.c. Pu is also unstable. This is in conflict with experiment and we are led to the conclusion that temperature effects must be of crucial importance for stabilizing cubic structures in U, Np, and Pu. Further, as a function of decreasing volume we predict a crystal structure sequence f.c.c. → b.c.t. → f.c.c. in Th, a sequence α-U → b.c.t. → b.c.c. in U, and a sequence β-Np → b.c.t. → b.c.c. in Np. Also, a sequence of transitions in Sc as a function of decreasing volume have been calculated, namely h.c.p. → f.c.c. → ω → β-Np → b.c.c.

Generalized cohesive energies and divalent-trivalent valence stabilities of the lanthanide metals have been investigated by means of total energy calculations using a full-potential linear muffin-tin orbital method in the local density approximation. For the localized 4f configuration we use experimental or estimated values for the spin and orbital polarization energies. The variation in the cohesive properties through the series is well accounted for, as is the general trend of the valence transition energies. A new view on the valence behaviour of the lanthanide metals is presented where the outstanding role of the spin polarization energy of the 4f electrons is clearly demonstrated. The elements with more than a half-filled 4f shell (n > 7, where n is the number of f-electrons) tend to decrease their f-count (i.e. increase the valence), while the elements with less than a half-filled 4f shell (n < 7) tend to increase their f-count (i.e. decrease the valence), in order to maximize the spin polarization energy. This, in conjunction with the well-known fact that the f-level binding energy increases with atomic number, explains the valence behaviour through the series. Using the experimental f → d promotion energies, transition pressures from the divalent to the trivalent state are calculated for Eu and Yb. The calculated pressures lie in the mixed valence pressure range observed experimentally.

We demonstrate that the conventional picture of crystal structure stabilities of transition metals, namely, as being determined by the degree of filling of the ``canonical'' d bands, breaks down at very high pressures. We show, by means of first-principles calculations, that for extreme compressions the pseudocore p states become broad and start to hybridize with the valence states. This results in a modification of the electronic structure and consequently unexpected crystal structures become stabilized. Simple model calculations using p- and d-canonical bands, which are allowed to hybridize with each other, predict that the bcc structure is the most stable crystallographic phase at high pressure for most of the transition elements, in agreement with our first-principles calculations.