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effective_mass(float mass_sq[], float *field_values)
As explained in section 6.3.2 the effective mass used for
initial conditions and for calculating spectra is given in program
units by

(4.17) 
This function calculates the grid average of
for each field and stores this square mass in the
array mass_sq. Once again, you can look at the example
file and easily see the equations from section 4.2.1. As with
the potential energy function the effective mass function accepts a
pointer to an array of field values. If this pointer is not NULL then
these values should be used instead of the field values on the
lattice.
In most of our model files the first part of the
effective_mass() function consists of the calculation of
the average squared values of the fields, either in terms of the
lattice values or the array of field values passed to the
function. Typically the derivatives
depend on these squared values. If this is the case for
your model then you can leave this part of the function unchanged. The
next part sets the square masses in terms of these values; these
equations are the part of the function unique to each model. Finally
the prefactor is added in. This last part should be the
same for all models.
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