The structures of expanded liquid alkali metals, Na. K, Rb and Cs, along the Liquid-vapour coexistence curve are investigated theoretically by employing the integral equation method in the modified hypernerted-chain (hIHNC) approximation and the molecular dynamics (m) simulation. Excellent agreement is obtained between the MHNC and the MD results, which also agree well with the experimental results. It is shown from lhese systematic investigations that the characteristic features of the density dependence of the structure alongthe liquid-vapour coexistence curve observed for expanded liquid Rb (Franz er01 1980) andCs (Winterelal1987) arecommon featuresforexpanded liquidalkali metals.

To investigate the role of the long-range oscillatory part of the effective pair potential in determining structural features of liquid metals, the structure factor S(q) and the pair correlation function g(r) calculated for the full pair potential are compared with those obtained for various cutoff forms of the pair potential. The calculations are performed by employing the modified hypernetted-chain approximation, which is one of the best approximations in the integral equation theory of liquid. The results for liquid cesium are presented in detail. It is shown that the peak positions of g(r) and S(q) are affected rather significantly by the long-range oscillatory tail of the pair potential, the contribution from the short-range attractive part being especially important. These features are quite different from the results for the Lennard-Jones liquid, whose structure is known to be dominantly determined by the short-range repulsive part of the pair potential.

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