Redefine the function f_calc to take nD as its second argument. (You could also make nD a global variable.) See below:
%% initial conditions
global d k0 h_bar ksi m E;
Ef = 2.77*10^3;
Kb = physconst('boltzmann'); % 1.38*10^(-23)
T = 0.12:0.24:6.4;
m = 9.1093837*10^(-31);
Tc = 1.2;
%t = T./Tc;
t = 0.1:0.1:2;
nD = floor(375./(2.*pi.*t.*1.2) - 0.5);
D = 10^(-8); % толщина пленки
ksi = 10^(-9);
%d = D/ksi;
d = 1000;
E = Ef/(pi*Kb*Tc);
h_bar = (1.0545726*10^(-34));
k0 = (ksi/h_bar)*sqrt(2.*m.*pi.*Kb.*Tc);
C_2 = 0;
for n = 0:49
C_2 = C_2 + (1/(2.*n+1)).*k0.*real(sqrt(3601+1i.*(2.*n+1))-((1+1i)./sqrt(2)).*sqrt(2.*n+1)); % константа
end
%% calculation
F = f_calc(t,nD);
plot(t,F, '-r');
%% F(t)
function F = f_calc(t,nD)
global d k0 h_bar ksi m;
F = 0;
for i = 1:20
n = nD(1,i);
F = F + 1/(2*n+1).*(k0.*real(((f_p1(n,t)-f_p2(n,t))./2))+(f_arg_2(n,t)-f_arg_1(n,t))./d);
end
F = -F;
%F = -(1/d).*F;
%F = F - C_2;
end
function p1 = f_p1(n,t)
p1 = ((1+1i)./sqrt(2)).*sqrt(t.*(2.*n+1));
end
function p2 = f_p2(n,t)
global E;
p2 = sqrt(3601+1i.*t.*(2.*n+1));
end
function n_lg = f_lg(n,t)
global d k0;
arg_of_lg = (1+exp(-1i*d*k0.*f_p1(n,t)))/(1+exp(-1i*d*k0.*f_p2(n,t)));
n_lg = log(abs(arg_of_lg));
end
function arg_1 = f_arg_1(n,t)
global d k0;
arg_1 = angle(1+exp(-1i*d*k0.*f_p1(n,t)));
end
function arg_2 = f_arg_2(n,t)
global d k0;
arg_2 = angle(1+exp(-1i*d*k0.*f_p2(n,t)));
end
