Birch–Murnaghan equation of state

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The Birch–Murnaghan isothermal equation of state, published in 1947 by Francis Birch of Harvard,[1] is a relationship between the volume of a body and the pressure to which it is subjected. This equation is named after Albert Francis Birch and Francis Dominic Murnaghan. Birch proposed this equation in a publication in 1947, based on the work of Murnaghan of Johns Hopkins University published in 1944.[2]

Expressions for the equation of state[edit]

The third-order Birch–Murnaghan isothermal equation of state is given by:

where P is the pressure, V0 is the reference volume, V is the deformed volume, B0 is the bulk modulus, and B0' is the derivative of the bulk modulus with respect to pressure. The bulk modulus and its derivative are usually obtained from fits to experimental data and are defined as


The expression for the equation of state is obtained by expanding the free energy f in the form of a series:

The internal energy, E(V), is found by integration of the pressure:

See also[edit]


  • ^ Birch, Francis (1947). "Finite Elastic Strain of Cubic Crystals". Physical Review. 71 (11): 809–824. Bibcode:1947PhRv...71..809B. doi:10.1103/PhysRev.71.809.
  • ^ Murnaghan, F. D. (1944). "The Compressibility of Media under Extreme Pressures". Proceedings of the National Academy of Sciences of the United States of America. 30 (9): 244–247. Bibcode:1944PNAS...30..244M. doi:10.1073/pnas.30.9.244. JSTOR 87468. PMC 1078704. PMID 16588651.