function F = force(r)
%r is the distance vector

global eps sigma bulk


%Lennard Jones
F = -24*eps*(2*(sigma^12)/(norm(r))^14 - (sigma^6)/(norm(r))^8).*r;

%Exp - 6
%F = -11*(-1.7227/(norm(r))^2 + .1609*exp(-1.7227*norm(r))*(1/norm(r) + 1.7227)).*r;

%Morse
%F = -eps*bulk/sigma*(exp(bulk*(1-norm(r)/sigma))-exp(bulk/2*(1-norm(r)/sigma)))/norm(r).*r;
