Possible 2S and 1D excited D and D s states are studied, the charmed states Dð2550Þ 0 , D Ã ð2600Þ, Dð2750Þ 0 , and D Ã ð2760Þ newly observed by the BABAR Collaboration are analyzed. The masses of these states are explored within the Regge trajectory phenomenology, and the strong decay widths are computed within the heavy-quark effective theory. Both the mass and the decay width indicate that Dð2550Þ 0 is a good candidate for 2 1 S 0 . The strong decay property of D Ã ð2600Þ and D Ã s1 ð2700Þ AE is described well by pure 2 3 S 1 states. If a mixing between 2 3 S 1 and 1 3 D 1 does exist, the mixing angle is not large and 2 3 S 1 is predominant. D Ã ð2760Þ and D Ã sJ ð2860Þ AE are possibly the 1 3 D 3 D, and D s , respectively. Dð2750Þ 0 and D Ã ð2760Þ seem two different states, and Dð2750Þ 0 is very possibly the 1Dð2 À ; 5 2 Þ though the possibility of 1Dð2 À ; 3 2 Þ has not been excluded. There may exist an unobserved meson D sJ ð2850Þ AE corresponding to D Ã sJ ð2860Þ AE .
We present an ab initio auxiliary field quantum Monte Carlo method for studying the electronic structure of molecules, solids, and model Hamiltonians at finite temperature. The algorithm marries the ab initio phaseless auxiliary field quantum Monte Carlo algorithm known to produce high accuracy ground state energies of molecules and solids with its finite temperature variant, long used by condensed matter physicists for studying model Hamiltonian phase diagrams, to yield a phaseless, ab initio finite temperature method. We demonstrate that the method produces internal energies within chemical accuracy of exact diagonalization results across a wide range of temperatures for HO (STO-3G), C (STO-6G), the one-dimensional hydrogen chain (STO-6G), and the multiorbital Hubbard model. Our method effectively controls the phase problem through importance sampling, often even without invoking the phaseless approximation, down to temperatures at which the systems studied approach their ground states and may therefore be viewed as exact over wide temperature ranges. This technique embodies a versatile tool for studying the finite temperature phase diagrams of a plethora of systems whose properties cannot be captured by a Hubbard U term alone. Our results moreover illustrate that the severity of the phase problem for model Hamiltonians far exceeds that for many molecules at all of the temperatures studied.
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