This list has been truncated:

SDM, LMNM, RN, RHDM, NDM, TN, SMM, LD, RDM,...

Supply exactly one missing member - there are a few to choose from.

Sloane did not provide any recognized sequence based on the number of letters, letter position, group or atomic number, yet if there is an intended sequence, a repeated pattern of the first letter of each can be possible intended...
S,L,R,R,N,T,
S,L,R,....
The pattern would indicate the next two could be
R, N, ... [RNM (Uranium), NPTNM (Neptunium)], though there is not a T (currently) that follows.