| Summary: | The transition metal dichalcogenide 1T-TaS_{2} is attracting growing attention because of the formation of rich density wave (DW) and superconducting transitions. However, the origin of the incommensurate DW state at the highest temperature (∼550K), which is “the parent state” of the rich physical phenomena, is still uncovered. Here, we present a natural explanation for the triple-q incommensurate DW in 1T-TaS_{2} based on the first-principles Hubbard model with on-site U. We apply the paramagnon interference mechanism that gives the nematic order in Fe-based superconductors. The derived order parameter has very unique characters: (i) an orbital-selective nature, and (ii) an unconventional sign reversal in both momentum and energy spaces. The present Letter will be useful for understanding the rich physics in 1T-TaS_{2}, 1T-VSe_{2}, and other transition metal dichalcogenides.
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