In‐Medium Similarity Renormalization Group Approach for Closed‐Shell Atoms

The in-medium similarity renormalization group (IMSRG) approach, based on a continuous unitary transformation, has been applied to closed-shell atoms. The flow equation, which is derived for the Hamiltonian, has been solved along with imaginary-time or White generators using the fourth-order Runge-Kutta and Magnus expansion methods. The behavior of the flow as a function of step size was investigated carefully. Our findings for ground state energy for the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mtext>He</mml:mtext></mml:mrow> <mml:annotation>$$ \mathrm{He} $$</mml:annotation></mml:semantics> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mtext>Ne</mml:mtext></mml:mrow> <mml:annotation>$$ \mathrm{Ne} $$</mml:annotation></mml:semantics> </mml:math> atoms from the IMSRG calculation are close to those obtained with full configuration interaction. Moreover, it has been observed that the IMSRG calculation based on the Magnus expansion approach, coupled with the White generator, requires the fewest steps to converge.