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The universal equations of state of solids recently proposed by several authors have been examined by comparing them with the theoretical results calculated by the augmentedplane-wave method and the quantum-statistical model proposed by Kalitkin and Kuz'mina from low to ultra-high pressures. It has been shown that the Vinet equation is in good agreement with the theoretical results both for the P -V relation and for the pressure dependence of the isothermal bulk modulus up to 10 TPa (V /V 0 = 0.20) for monatomic solids and up to 1 TPa (V /V 0 = 0.35) for diatomic solids. The Kumari-Dass and the Dodson equations become less successful below V /V 0 = 0.7 if the zero-pressure values for B 0 , B 0 and B 0 are used. For monatomic solids the Holzapfel equation has a very similar structure to that of the Vinet equation at low and medium compressions and it is in good agreement with the theoretical values up to ultra-high pressures.For the application to polyatomic solids a remedy for the shortcomings of the Vinet equation at very high pressures is given on the basis of the quantum-statistical model. The resulting equation is in good agreement with the theoretical values from low to ultra-high pressures both for monatomic and for diatomic solids.

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The equations of state (EOS) of solid LiH in the 81 (NaCl-type) and K (CsCl-type) structures have been calculated within the local-density approximation (LDA), using the augmented-planewave method with Ceperley-Alder's form for the exchange-correlation energy. Using the EOS obtained by the LDA, the volume dependencies of the Debye temperatures for both phases have been determined. The calculated Debye temperatures in the 81 phase at zero pressure and zero temperature are 1246 K for LiH and 972 K for LiD in agreement with the experimental values in the T =0 K limit at normal pressure which are 1190+80 K for LiH and 1030+50 K for LiD, respectively. The calculated lattice constants for isotopic LiH at zero pressure are in agreement with the experimental values at T = 83 K by 1.9/o. With these results the shock-wave equations of state (the Hugoniot) for both phases have been calculated. The Hugoniots for the 81 phase are in agreement with the experiment by Marsh.

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