External module function

In order to run *mcphas*,*singleion*
the following function has to be present in the module file **.so*:

extern "C" void Icalc(Vector & I,double * T, Vector & Hxc, Vector & Hext, double * gJ, Vector & MODPAR, char ** sipffilename, double * lnZ,double * U, ComplexMatrix & Icalc_parstorage);

Note for windows users with MINGW the declaration should be *extern "C" __declspec(dllexport) void
Icalc(...)*.

The meaning of the symbols is as follows:

on input T temperature[K] Hxc vector of exchange field [meV] (n-dimensional, for a set of n operators I) Hext external magnetic field [T] (3 components) gJ Lande factor as read from sipf file MODPAR Vector with Parameters read from single ion property file sipffilename file name of the single ion parameter file Icalc_parstorage parameter matrix (initialized by Icalc_parameter_storage_matrix_init) it should/may contain any information, e.g. population numbers of the states (imaginary part of row 0) and eigenvalues (real part of row 0) with values set by the most recent call for this ion (use of this matrix is optional, it is provided to save time by storing here information instead of recalculating it for every call of Icalc) on output I thermal expectation value of operator <I> (n- dimensional with n>=1, may be an arbitrary set of operators, lnZ natural logarithm of single ion partition function U single ion magnetic energy [meV] Icalc_parstorage parameter matrix matrix (optional) it should/may contain any information for the next call of Icalc, e.g. population numbers of the states (imaginary part of row 0) and eigenvalues (real part of row 0) ...The module function must perform the following tasks:

- check if the dimensions of vectors
and xc
(taken by
*mcphas*from the number of interaction constant columns in*mcphas.j*) and MODPAR (taken by*mcphas*from the number of params in the single ion property file) agree with the module specifications. Note a module may be designed to take different dimensions depending on the input files, however the dimensions of vectors and xc has to agree and must be within the range of dimensions which can be treated by the module. If the check fails the module function should exit the program with an appropriate error message - the module should calculate from external field at a given temperature the thermal expectation values of the operators and return them as a vector . Input file parameters params are supplied as a vector MODPAR and Lande factor as gJ and can be used for this purpose. The Hamiltonian is usually assumed to be of the general form .
- the natural logarithm of the partition sum Z should be calculated and returned as lnZ,
- the magnetic energy U should be calculated and returned as U,
- if all quantities should be evaluated assuming that all Boltzmann probabilities are zero except for the state number , for which the probability .

... as an example the anisotropic doublet function is given as a
loadable module in the file *./examples/cecu2a/1ion_mod/kramer.c*, in the same
directory a Makefile is given in order to show how this loadable
module is compiled (for details see appendix A).

Another more complicated example, the calculation of the magnetisation in a tetragonal quasi-quartet system is given in Appendix C.

Martin Rotter 2017-01-10