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We present calculations of the optical response of the DNA bases and base pairs both in their normal and tautomeric forms in the gas phase, using time-dependent density functional theory (TDDFT). These calculations are performed in real time within the adiabatic approximation with a basis of local orbitals. Our results for the individual bases are in good agreement with experiment and computationally more demanding calculations of chemical accuracy. The optical response of base pairs indicates that the differences between normal and tautomeric forms in certain cases are significant enough to provide a means of identification.

We report on a general method for the calculation of the frequency-dependent optical response of clusters based upon time-dependent density functional theory (TDDFT). The implementation is done using explicit propagation in the time domain and a self-consistent program that uses a linear combination of atomic orbitals (LCAO). Our actual calculations employ the SIESTA program, which is designed to be fast and accurate for large clusters. We use the adiabatic local density approximation to account for exchange and correlation effects. Results are presented for the imaginary part of the linear polarizability, $\mathrm{Im}\ensuremath{\alpha}(\ensuremath{\omega}),$ and the dipole strength function, $S(\ensuremath{\omega}),$ of ${\mathrm{C}}_{60}$ and ${\mathrm{Na}}_{8},$ and compared to previous calculations and to experiment. We also develop a method for the calculation of the integrated frequency-dependent second-order nonlinear polarizability for the case of a step function electric field, ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\gamma}}}_{\mathrm{step}}(\ensuremath{\omega}),$ and present results for ${\mathrm{C}}_{60}.$