密度怎么没和压力关联？

兰式折射

用P-R方程，编的密度函数，看了看结果，发现密度没能和压力关联上，请教大家了，谢谢。
#include "udf.h"
#define Rg 8.314
#define PI 3.141592654
#define Mw 44.01
#define omiga 0.225
#define Pc 7.3834e6
#define Tc 304.21
#define Zc 0.2769
#define Vc 2.15517e-3
#define NRK 0.77
#define CC1 453.577
#define CC2 1.65014
#define CC3 -1.24814e-3
#define CC4 3.78201e-7
#define CC5 0.00

DEFINE_PROPERTY(density,cell,thread)
{
real P,T,ac,at,b,m,v,afun;
real a1,a2,a3;
real vv;
real vv1,vv2,vv3;
real qq,qq3,sqq,rr,tt,dd;
P=C_P(cell,thread)+101325;
T=C_T(cell,thread);
ac=0.45727*Rg*Rg*Tc*Tc/Pc;
b=0.0778*Rg*Tc/Pc;
m=0.37464+1.54226*omiga-0.26992*pow(omiga,2);
at=pow(1+m*(1-pow(T/Tc,0.5)),2);
afun=ac*at;
P=Rg*T/(v-b)-afun/(v*(v+b)+b*(v-b));
a1=b-Rg*T/P;
a2=afun/P-3*b*b-2*Rg*b*T/P;
a3=b*b*b+Rg*b*b*T/P-b*afun/P;
qq=(a1*a1-3.*a2)/9.;
rr=(2*a1*a1*a1-9.*a1*a2+27.*a3)/54.;
qq3=qq*qq*qq;
dd=qq3-rr*rr;
if(dd<0.)
{
tt=sqrt(-dd)+pow(fabs(rr),0.333333);
vv=(tt+qq/tt)-a1/3.;
}
else
{
tt=acos(rr/sqrt(qq3));
sqq=sqrt(qq);
vv1=-2.*sqq*cos(tt/3.)-a1/3.;
vv2=-2.*sqq*cos((tt+2.*PI)/3.)-a1/3.;
vv3=-2.*sqq*cos((tt+4.*PI)/3.)-a1/3.;
vv=(vv1>vv2)?vv1:vv2;
vv=(vv>vv3)?vv:vv3;
}
return 1./vv;
}