Mean field approximation, high pressure

This perturbative expression for the attractive force shift is derived from a van der Waals mean field approximation (23). Although the predictions of this model have been found to agree with numerous high pressure vibrational frequency shift measurements (23,25,28), a non-linear attractive force model has recently been suggested to be appropriate for some systems (26,27). 

Pressure variation of the electric field gradient (e.f.g.) in highly pure powdered samples of As and Sb has been followed up to 2.0 GPa at 293 K via pulsed NQR determination of the appropriate As, Sb or Sb resonance frequencies. The results were compared with theoretical calculations of the total e.f.g. by means of full-potential linearized augmented plane wave (FLAPW) calculations. The theoretical values were approximately 70% of the measured ones, but comparison was limited to some extent by the accuracy of the values used for the quadrupolar moments of As and Sb. Additional uncertainties arose from the... 

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