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from umatgen import *
b_in=10
GST=umat(Implicit=True, project='Tets_GST',
outfile="GST", outroutine="GST", lang='F95')
C=GST.C
# calculate the GST invariant
E_i=I4_GST(C, b=b_in)
var('parK1, parK2, parK')
W=parK1/(2*parK2)*(exp(parK2*(E_i)^2)-1)
# Setting the strain energy function
GST.W=W
# Output the source code
GST.write()