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Structure of the Particle-Hole Amplitudes in No-core Shell Model Wave Functions

We study the structure of the no-core shell model wave functions for $^6$Li and $^{12}$C by investigating the ground state and first excited state electron scattering charge form factors. In both nuclei, large particle-hole ($ph$) amplitudes in the wave functions appear with the opposite sign to that needed to reproduce the shape of the $(e,e')$ form factors, the charge radii, and the B(E2) values for the lowest two states. The difference in sign appears to arise mainly from the monopole $Δ\hbarω=2$ matrix elements of the kinetic and potential energy (T+V) that transform under the harmonic oscillator SU(3) symmetries as $(λ,μ)=(2,0)$. These are difficult to determine self-consistently, but they have a strong effect on the structure of the low-lying states and on the giant monopole and quadrupole resonances. The Lee-Suzuki transformation, used to account for the restricted nature of the space in terms of an effective interaction, introduces large higher-order $Δ\hbarω=n, n>$2, $ph$ amplitudes in the wave functions. The latter $ph$ excitations aggravate the disagreement between the experimental and predicted $(e,e')$ form factors with increasing model spaces, especially at high momentum transfers. For sufficiently large model spaces the situation begins to resolve itself for $^6$Li, but the convergence is slow. A prescription to constrain the $ph$ excitations would likely accelerate convergence of the calculations.