There are several DFT based commercial or open source softwares that could be used to simulate the optical properties of nanomaterials, particularly metallic nanoparticles. Basically, what you are looking for is calculating the susceptibility tensor from Kubo-Greenwood relation:
$$\chi_{ij}(\omega) = \frac{e^{2}}{\hbar m_{e}^{2} V} \sum_{n,m,\mathbf{k}} \frac{f_{m,\mathbf{k}} - f_{n,\mathbf{k}}}{\omega_{nm}^{2}(\mathbf{k}) (\omega_{nm}(\mathbf{k})-\omega-i\frac{\Gamma}{\hbar})}p_{nm}^{i}(\mathbf{k})p_{mn}^{j}(\mathbf{k})$$
Where $p_{nm}^{i}(\mathbf{k}) = \langle n\mathbf{k} | \mathbf{p}^{i} | m\mathbf{k}\rangle$ is i-th component of the momentum operator between states n and m. $m_{e}$ is the electron mass, $e$ is electron charge, $V$ is the volume, $\Gamma$ the energy broadening, $\hbar\omega_{nm}(\mathbf{k})= E_{n}(\mathbf{k}) - E_{m}(\mathbf{k})$, and finally $f_{n,\mathbf{k}}$ is the Fermi function evaluated at the band energy of $E_{n}(\mathbf{k})$.
Relative dielectric constant ($\epsilon_{r}$), polarizability ($\alpha$), and optical conductivity ($\sigma$) are related to susceptibility as:
$$\epsilon_{r}(\omega) = 1 + \chi(\omega)$$
$$\alpha(\omega) = V \epsilon_{0} \chi(\omega)$$
$$\sigma(\omega) = -i \omega \epsilon_{0} \chi(\omega)$$
Finally, the refractve indices are derived from relative dielectric constant ($\epsilon_{r}$) as:
$$n(\omega) + i\kappa(\omega) = \sqrt{\epsilon_{r}(\omega)}$$
QuantumATK
An example of commercial software that could be used to extract these optical properties for metallic nanoparticles is QuantumATK.
