pute the compressibility coefficients
        function [ccc, ccg, cgc,cgg] = compressibilityCoeffsPgPc(this,Sl,pl,pg)
            % Get porous media parameters
            phi  = this.porousMedia.phi;
            % Get fluid densities
            rhol = this.liquidFluid.getDensity(pl);
            rhog = this.gasFluid.getDensity(pg);
            % Capillary pressure
            pc = pg - pl;
            % Derivative of the liquid saturation degree wrt pc
            dSldpc = this.capillaryPressure.derivativ