The Landau free energy of a model solid with an anharmonic potential (often a double well) at each site and mean-field like intersite coupling has been calculated both analytically and from molecular dynamics simulation. The calculated free energy function is not well described by a simple polynomial in the order parameter. This result is not due to critical fluctuations in the Ginzburg interval. If, however, such a polynomial is used the coefficient of the fourth order term is found to be highly temperature dependent. For a certain range of model parameters this coefficient is small relative to that of the second order term. These observations help to explain the occurrence of 'non-critical, non-standard' values of the exponent f3 in the variation of the order parameter, Q, with temperature: Q Q (T, - T)S. More importantly they also help to explain why so man natural systems behave in a tricritical-type manner, with f3- Y /4. © 1990, Taylor & Francis Group, LLC. All rights reserved.
