Summary This code seeks to find the geometry that has the minimum energy under the constrain that its softest phonon mode(s) has zero frequency. The algorithm is described in: A. van de Walle, S. Kadkhodaei, R. Sun, and Q.-J. Hong., 'Epicycle method for elasticity limit calculations.', Phys. Rev. B, 95:144113, 2017. http://dx.doi.org/10.1103/PhysRevB.95.144113 A theoretical motivation for using such a quantity can be found in: A. van de Walle, Q. Hong, S. Kadkhodaei, and R. Sun., 'The free energy of mechanically unstable phases', Nature Commun. 6, 7559 (2015). http://dx.doi.org/10.1038/ncomms8559 This is code often invoked automatically within the script robustrelax_xxxx with the -id option. In this case, all options are passed with -idop \"[any of the options described below]\". Also, all files described below then reside in the 01 subdirectory and the log file is 01/infdet.log . It is implemented as two nested optimization problems: -an inner optimization problem to find the softest