Hydrogen molecules, compression

The system first used to introduce the quantum definition of pressure consists of a linear chain of hydrogen molecules compressed between a pair of Ne atoms [12]. The vise is depicted in Figure 7 for the compression of five H2 molecules, the system Ne Hio Ne. The calculations were performed at the SCF/SCVS level using the 6-311G basis. The effect of electron correlation on the calculated pressure and on the variation of properties with pressure is found to be minimal, amounting to a few per cent, as determined in... 

The presence of intermolecular forces also accounts for the variation in the compression factor. Thus, for gases under conditions of pressure and temperature such that Z > 1, the repulsions are more important than the attractions. Their molar volumes are greater than expected for an ideal gas because repulsions tend to drive the molecules apart. For example, a hydrogen molecule has so few electrons that the its molecules are only very weakly attracted to one another. For gases under conditions of pressure and temperature such that Z < 1, the attractions are more important than the repulsions, and the molar volume is smaller than for an ideal gas because attractions tend to draw molecules together. To improve our model of a gas, we need to add to it that the molecules of a real gas exert attractive and repulsive forces on one another. 

Gas hydrates are crystalline compounds in which smaller gas molecules (<0.9 nm) are encaged inside the lattices of hydrogen-bonded ice crystals. The gas molecules in the crystalline solids are effectively compressed, volumetrically, by a factor of 164. A comprehensive review of the physical and chemical properties of gas hydrates can be found in Sloan (1998, 2003). 

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