double precision function ec_scan(rho_a,rho_b,tau,grad_rho_2) include 'constants.include.F' implicit none double precision, intent(in) :: rho_a,rho_b,tau,grad_rho_2 double precision :: cst_13,cst_23,cst_43,cst_53,rho_inv,cst_18,cst_3pi2 double precision :: thr,nup,ndo,xi,s,spin_d,drho,drho2,rho,inv_1alph,e_c_lsda1,h0 double precision :: rs,t_w,t_unif,ds_xi,alpha,fc_alpha,step_f,cst_1alph,beta_inf double precision :: c_1c,c_2c,d_c,e_c_ldsa1,h1,phi,t,beta_rs,gama,a,w_1,g_at2,phi_3,e_c_1 double precision :: b_1c,b_2c,b_3c,dx_xi,gc_xi,e_c_lsda0,w_0,g_inf,cx_xi,x_inf,f0,e_c_0 thr = 1.d-12 nup = max(rho_a,thr) ndo = max(rho_b,thr) rho = nup + ndo ec_scan = 0.d0 if((rho).lt.thr)return ! constants ... rho_inv = 1.d0/rho cst_13 = 1.d0/3.d0 cst_23 = 2.d0 * cst_13 cst_43 = 4.d0 * cst_13 cst_53 = 5.d0 * cst_13 cst_18 = 1.d0/8.d0 cst_3pi2 = 3.d0 * pi*pi drho2 = max(grad_rho_2,thr) drho = dsqrt(drho2) if((nup-ndo).gt.0.d0)then spin_d = max(nup-ndo,thr) else spin_d = min(nup-ndo,-thr) endif c_1c = 0.64d0 c_2c = 1.5d0 d_c = 0.7d0 b_1c = 0.0285764d0 b_2c = 0.0889d0 b_3c = 0.125541d0 gama = 0.031091d0 ! correlation energy lsda1 call ec_only_lda_sr(0.d0,nup,ndo,e_c_lsda1) xi = spin_d/rho rs = (cst_43 * pi * rho)**(-cst_13) s = drho/( 2.d0 * cst_3pi2**(cst_13) * rho**cst_43 ) t_w = drho2 * cst_18 * rho_inv ds_xi = 0.5d0 * ( (1.d0+xi)**cst_53 + (1.d0 - xi)**cst_53) t_unif = 0.3d0 * (cst_3pi2)**cst_23 * rho**cst_53*ds_xi t_unif = max(t_unif,thr) alpha = (tau - t_w)/t_unif cst_1alph= 1.d0 - alpha if(cst_1alph.gt.0.d0)then cst_1alph= max(cst_1alph,thr) else cst_1alph= min(cst_1alph,-thr) endif inv_1alph= 1.d0/cst_1alph phi = 0.5d0 * ( (1.d0+xi)**cst_23 + (1.d0 - xi)**cst_23) phi_3 = phi*phi*phi t = (cst_3pi2/16.d0)**cst_13 * s / (phi * rs**0.5d0) w_1 = dexp(-e_c_lsda1/(gama * phi_3)) - 1.d0 a = beta_rs(rs) /(gama * w_1) g_at2 = 1.d0/(1.d0 + 4.d0 * a*t*t)**0.25d0 h1 = gama * phi_3 * dlog(1.d0 + w_1 * (1.d0 - g_at2)) ! interpolation function fc_alpha = dexp(-c_1c * alpha * inv_1alph) * step_f(cst_1alph) - d_c * dexp(c_2c * inv_1alph) * step_f(-cst_1alph) ! first part of the correlation energy e_c_1 = e_c_lsda1 + h1 dx_xi = 0.5d0 * ( (1.d0+xi)**cst_43 + (1.d0 - xi)**cst_43) gc_xi = (1.d0 - 2.3631d0 * (dx_xi - 1.d0) ) * (1.d0 - xi**12.d0) e_c_lsda0= - b_1c / (1.d0 + b_2c * rs**0.5d0 + b_3c * rs) w_0 = dexp(-e_c_lsda0/b_1c) - 1.d0 beta_inf = 0.066725d0 * 0.1d0 / 0.1778d0 cx_xi = -3.d0/(4.d0*pi) * (9.d0 * pi/4.d0)**cst_13 * dx_xi x_inf = 0.128026d0 f0 = -0.9d0 g_inf = 1.d0/(1.d0 + 4.d0 * x_inf * s*s)**0.25d0 h0 = b_1c * dlog(1.d0 + w_0 * (1.d0 - g_inf)) e_c_0 = (e_c_lsda0 + h0) * gc_xi ec_scan = e_c_1 + fc_alpha * (e_c_0 - e_c_1) end double precision function step_f(x) implicit none double precision, intent(in) :: x if(x.lt.0.d0)then step_f = 0.d0 else step_f = 1.d0 endif end double precision function beta_rs(rs) implicit none double precision, intent(in) ::rs beta_rs = 0.066725d0 * (1.d0 + 0.1d0 * rs)/(1.d0 + 0.1778d0 * rs) end