exp(1+(1/$T)*(($T-1)/1)-($T*$T)/(1*1)*exp((1/$T)*(($T-1)/1)*(1-2*$T)) )

assume T_M = 400 K      E = 2eV

Grace:
exp( 1 + (2/(8.617e-5*$T))*(($T-400)/400) - ($T*$T)/(400*400) * exp((2/(8.617e-5*$T))*(($T-400)/400)*(1-(2*8.617e-5*$T/2)))-(2*8.617e-5*400/2))

Octave / MATLAB:
kernel_y = exp( 1 + (2./(8.617e-5.*kernel_x)).*((kernel_x-400)./400) - (kernel_x.*kernel_x)./(400*400) .* exp((2./(8.617e-5.*kernel_x)).*((kernel_x-400)./400).*(1-(2.*8.617e-5.*kernel_x./2)))-(2*8.617e-5*400/2));