static double
bcc_Zq(double qa, double qb, double qc, double lattice_spacing, double lattice_distortion)
{
    // Equations from Matsuoka 26-27-28, multiplied by |q|
    const double a1 = (-qa + qb + qc)/2.0;
    const double a2 = (+qa - qb + qc)/2.0;
    const double a3 = (+qa + qb - qc)/2.0;

#if 1
    // Matsuoka 29-30-31
    //     Z_k numerator: 1 - exp(a)^2
    //     Z_k denominator: 1 - 2 cos(d a_k) exp(a) + exp(2a)
    // Rewriting numerator
    //         => -(exp(2a) - 1)
    //         => -expm1(2a)
    // Rewriting denominator
    //         => exp(a)^2 - 2 cos(d ak) exp(a) + 1)
    //         => (exp(a) - 2 cos(d ak)) * exp(a) + 1
    const double arg = -0.5*square(lattice_spacing*lattice_distortion)*(a1*a1 + a2*a2 + a3*a3);
    const double exp_arg = exp(arg);
    const double Zq = -cube(expm1(2.0*arg))
        / ( ((exp_arg - 2.0*cos(lattice_spacing*a1))*exp_arg + 1.0)
          * ((exp_arg - 2.0*cos(lattice_spacing*a2))*exp_arg + 1.0)
          * ((exp_arg - 2.0*cos(lattice_spacing*a3))*exp_arg + 1.0));

#elif 0
    // ** Alternate form, which perhaps is more approachable
    //     Z_k numerator   => -[(exp(2a) - 1) / 2.exp(a)] 2.exp(a)
    //                     => -[sinh(a)] exp(a)
    //     Z_k denominator => [(exp(2a) + 1) / 2.exp(a) - cos(d a_k)] 2.exp(a)
    //                     => [cosh(a) - cos(d a_k)] 2.exp(a)
    //     => Z_k = -sinh(a) / [cosh(a) - cos(d a_k)]
    //            = sinh(-a) / [cosh(-a) - cos(d a_k)]
    //
    // One more step leads to the form in sasview 3.x for 2d models
    //            = tanh(-a) / [1 - cos(d a_k)/cosh(-a)]
    //
    const double arg = 0.5*square(lattice_spacing*lattice_distortion)*(a1*a1 + a2*a2 + a3*a3);
    const double sinh_qd = sinh(arg);
    const double cosh_qd = cosh(arg);
    const double Zq = sinh_qd/(cosh_qd - cos(lattice_spacing*a1))
                    * sinh_qd/(cosh_qd - cos(lattice_spacing*a2))
                    * sinh_qd/(cosh_qd - cos(lattice_spacing*a3));
#else
    const double arg = 0.5*square(lattice_spacing*lattice_distortion)*(a1*a1 + a2*a2 + a3*a3);
    const double tanh_qd = tanh(arg);
    const double cosh_qd = cosh(arg);
    const double Zq = tanh_qd/(1.0 - cos(lattice_spacing*a1)/cosh_qd)
                    * tanh_qd/(1.0 - cos(lattice_spacing*a2)/cosh_qd)
                    * tanh_qd/(1.0 - cos(lattice_spacing*a3)/cosh_qd);
#endif

    return Zq;
}


// occupied volume fraction calculated from lattice symmetry and sphere radius
static double
bcc_volume_fraction(double radius, double lattice_spacing)
{
    return 2.0*sphere_volume(radius/lattice_spacing);
}

static double
form_volume(double radius)
{
    return sphere_volume(radius);
}


static double Iq(double q, double lattice_spacing,
    double lattice_distortion, double radius,
    double sld, double solvent_sld)
{
    // translate a point in [-1,1] to a point in [0, 2 pi]
    const double phi_m = M_PI;
    const double phi_b = M_PI;
    // translate a point in [-1,1] to a point in [0, pi]
    const double theta_m = M_PI_2;
    const double theta_b = M_PI_2;

    double outer_sum = 0.0;
    for(int i=0; i<GAUSS_N; i++) {
        double inner_sum = 0.0;
        const double theta = GAUSS_Z[i]*theta_m + theta_b;
        double sin_theta, cos_theta;
        SINCOS(theta, sin_theta, cos_theta);
        const double qc = q*cos_theta;
        const double qab = q*sin_theta;
        for(int j=0;j<GAUSS_N;j++) {
            const double phi = GAUSS_Z[j]*phi_m + phi_b;
            double sin_phi, cos_phi;
            SINCOS(phi, sin_phi, cos_phi);
            const double qa = qab*cos_phi;
            const double qb = qab*sin_phi;
            const double form = bcc_Zq(qa, qb, qc, lattice_spacing, lattice_distortion);
            inner_sum += GAUSS_W[j] * form;
        }
        inner_sum *= phi_m;  // sum(f(x)dx) = sum(f(x)) dx
        outer_sum += GAUSS_W[i] * inner_sum * sin_theta;
    }
    outer_sum *= theta_m;
    const double Zq = outer_sum/(4.0*M_PI);
    const double Pq = sphere_form(q, radius, sld, solvent_sld);
    return bcc_volume_fraction(radius, lattice_spacing) * Pq * Zq;
}


static double Iqabc(double qa, double qb, double qc,
    double lattice_spacing, double lattice_distortion, double radius,
    double sld, double solvent_sld)
{
    const double q = sqrt(qa*qa + qb*qb + qc*qc);
    const double Zq = bcc_Zq(qa, qb, qc, lattice_spacing, lattice_distortion);
    const double Pq = sphere_form(q, radius, sld, solvent_sld);
    return bcc_volume_fraction(radius, lattice_spacing) * Pq * Zq;
}