/*gbar0 compute g-bar gaunt factor for neutrals */
#include "cddefines.h"
#include "phycon.h"
#include "gbar0.h"

void gbar0(double ex, 
	  float *g)
{
	double a, 
	  b, 
	  c, 
	  d, 
	  y;

#	ifdef DEBUG_FUN
	fputs( "<+>gbar0()\n", debug_fp );
#	endif

	/* written by Dima Verner
	 *
	 * Calculation of the effective Gaunt-factor by use of 
	 * Van Regemorter (1962, ApJ 136, 906) fits for neutrals
	 *  Input parameters: 
	 * ex - energy ryd - now K
	 * t  - temperature in K
	 *  Output parameter:
	 * g  - effective Gaunt factor
	 * */

	/* y = ex*157813.7/te */
	y = ex/phycon.te;
	if( y < 0.01 )
	{
		*g = (float)(0.29*(log(1.0+1.0/y) - 0.4/POW2(y + 1.0))/exp(y));
	}
	else
	{
		if( y > 10.0 )
		{
			*g = (float)(0.066/sqrt(y));
		}
		else
		{
			a = 1.5819068e-02;
			b = 1.3018207e00;
			c = 2.6896230e-03;
			d = 2.5486007e00;
			d = log(y/c)/d;
			*g = (float)(a + b*exp(-0.5*POW2(d)));
		}
	}


#	ifdef DEBUG_FUN
	fputs( " <->gbar0()\n", debug_fp );
#	endif
	return;
}