  We present a new and efficient implementation of the closed shell coupled
cluster singles and doubles with perturbative triples method (CC3) in the
electronic structure program $e^T$. Asymptotically, a ground state calculation
has an iterative cost of $4n_{\text{V}}^{4}n_{\text{O}}^{3}$ floating point
operations (FLOP), where $n_{\text{V}}$ and $n_{\text{O}}$ are the number of
virtual and occupied orbitals respectively. The Jacobian and transpose Jacobian
transformations, required to iteratively solve for excitation energies and
transition moments, both require $8n_{\text{V}}^{4}n_{\text{O}}^{3}$ FLOP. We
have also implemented equation of motion (EOM) transition moments for CC3. The
EOM transition densities require recalculation of triples amplitudes, as
$n_{\text{V}}^{3}n_{\text{O}}^{3}$ tensors are not stored in memory. This
results in a noniterative computational cost of
$10n_{\text{V}}^{4}n_{\text{O}}^{3}$ FLOP for the ground state density and
$26n_{\text{V}}^{4}n_{\text{O}}^{3}$ FLOP per state for the transition
densities. The code is compared to the CC3 implementations in CFOUR, Dalton and
Psi4. We demonstrate the capabilities of our implementation by calculating
valence and core excited states of L-proline.
