  A reduced-complexity variant of equation-of-motion coupled-cluster singles
and doubles (EOM-CCSD) method is formulated in terms of state-averaged excited
state pair natural orbitals (PNO) designed to describe manifolds of excited
states. State-averaged excited state PNOs for the {\em target} manifold are
determined by averaging CIS(D) pair densities over the computational manifold.
To assess the performance of PNO-EOM-CCSD approach on extended systems the new
massively parallel canonical EOM-CCSD program has been developed in the
Massively Parallel Quantum Chemistry program that allows treatment of systems
with 50+ atoms using realistic basis sets with 1000+ functions. The use of
state-averaged PNOs offers several potential advantages relative to the
recently proposed state-specific PNOs: our approach is robust with respect to
root flipping and state degeneracies, it is more economical when computing
large manifolds of states, and it simplifies evaluation of transition-specific
observables such as dipole moments. With the PNO truncation threshold of
$10^{-7}$, the errors in excitation energies are on average below 0.02 eV for
the first six singlet states of 28 organic molecules included in the standard
test set of Thiel and co-workers (J. Chem. Phys. 2008, 128, 134110) with 50-70
state-averaged PNOs per pair.
