You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: evaluatePressure.m
    path of file: ./repos/M-SPARC/src
    
    the code of the file until where you have to start completion: function pressure = evaluatePressure(S)
% @brief    Function to calculate pressure in periodic systems (O(N^3))
% @authors
%          Abhiraj Sharma <asharma424@gatech.edu>
%          Phanish Suryanarayana <phanish.suryanarayana@ce.gatech.edu>
%
% @references
%             "On the calculation of the stress tensor in real-space Kohn-Sham
%              density functional theory (Sharma et. al. 2018)"
%
% @copyright (c) 2019 Material Physics & Mechanics Group, Georgia Tech
%=================================================================================


    
function pressure = evaluatePressure(S)
% @brief    Function to calculate pressure in periodic systems (O(N^3))
% @authors
%          Abhiraj Sharma <asharma424@gatech.edu>
%          Phanish Suryanarayana <phanish.suryanarayana@ce.gatech.edu>
%
% @references
%             "On the calculation of the stress tensor in real-space Kohn-Sham
%              density functional theory (Sharma et. al. 2018)"
%
% @copyright (c) 2019 Material Physics & Mechanics Group, Georgia Tech
%=================================================================================

% Component from Energy related terms calculated explicitly
Dphi_x = S.grad_1*(S.phi);
Dphi_y = S.grad_2*(S.phi);
Dphi_z = S.grad_3*(S.phi);

P_eng = 0.0;
ks = 1;
for spin = 1:S.nspin
	for kpt = 1:S.tnkpt
		P_eng = P_eng - S.occfac*2*S.wkpt(kpt)*sum(S.EigVal(:,ks).*S.occ(:,ks)) ;
		ks = ks + 1;
	end