  With the help of our recently developed massively parallel DGDFT
(Discontinuous Galerkin Density Functional Theory) methodology, we perform
large-scale Kohn-Sham density functional theory calculations on phosphorene
nanoribbons with armchair edges (ACPNRs) containing a few thousands to ten
thousand atoms. The use of DGDFT allows us to systematically achieve
conventional plane wave basis set type of accuracy, but with a much smaller
number (about 15) of adaptive local basis (ALB) functions per atom for this
system. The relatively small number degrees of freedom required to represent
the Kohn-Sham Hamiltonian, together with the use of the pole expansion the
selected inversion (PEXSI) technique that circumvents the need to diagonalize
the Hamiltonian, result in a highly efficient and scalable computational scheme
for analyzing the electronic structures of ACPNRs as well as its dynamics. The
total wall clock time for calculating the electronic structures of large-scale
ACPNRs containing 1080-10800 atoms is only 10-25 s per self-consistent field
(SCF) iteration, with accuracy fully comparable to that obtained from
conventional planewave DFT calculations. For the ACPNR system, we observe that
the DGDFT methodology can scale to 5,000-50,000 processors. We use DGDFT based
ab-initio molecular dynamics (AIMD) calculations to study the thermodynamic
stability of ACPNRs. Our calculations reveal that a 2 * 1 edge reconstruction
appears in ACPNRs at room temperature.
