Enabled zero-valued twobody Tersoff parameters

    params[nparams].lam1 = atof(words[13]);

    params[nparams].biga = atof(words[14]);

    if (params[nparams].lam3 < 0.0 || params[nparams].c <= 0.0 || 

	params[nparams].d <= 0.0 || params[nparams].powern <= 0.0 || 

    if (params[nparams].lam3 < 0.0 || params[nparams].c < 0.0 || 

	params[nparams].d < 0.0 || params[nparams].powern < 0.0 || 

	params[nparams].beta < 0.0 || params[nparams].lam2 < 0.0 || 

	params[nparams].bigb < 0.0 || params[nparams].bigr < 0.0 ||

	params[nparams].bigd < 0.0 ||

	params[nparams].bigd >= params[nparams].bigr ||

	params[nparams].bigd > params[nparams].bigr ||

	params[nparams].lam3 < 0.0 || params[nparams].biga < 0.0)

      error->all("Illegal Tersoff parameter");