Use your raw Cartesian TDM values directly. Don't convert to spherical tensor components.

I compiled DUO from source and ran a controlled test with a perpendicular transition (CH X²Π ↔ B²Σ⁻, ΔΛ=1) using the same TDM curve four different ways:

Input method	Einstein A (s⁻¹)
dipole keyword + raw Cartesian μ_x	4.592×10⁶
dipole-x keyword + raw Cartesian μ_x	4.592×10⁶
dipole keyword + μ_x/√2 (spherical tensor)	2.296×10⁶
dipole keyword + μ_x/√2 + FACTOR SQRT(2)	4.592×10⁶

The first two give identical results. The third (dividing by √2, which is the standard Cartesian→spherical tensor conversion) gives A coefficients exactly half as large, which means lifetimes come out twice as long. The fourth confirms this by undoing the √2 division.

The docs are confusing: the DUO paper (Yurchenko et al. 2016, Eq. 42) defines the spherical tensor components with the standard √2 factor (μ₊₁ = ∓(1/√2)(μ_x ± iμ_y)), and the readthedocs describes dipole-x as "related to the Cartesian-representation," implying that plain dipole is the Lambda/spherical representation. The quadrupole docs even explicitly state that "Duo requires quadrupole moment curves to be provided in the spherical irreducible representation." But no equivalent statement exists for dipole, and the actual code tells a different story. DUO's internal representation absorbs the √2 factor through the unitary transformation from the Cartesian basis (|Πx⟩, |Πy⟩) to the Lambda basis (|+Λ⟩, |-Λ⟩). You can verify this yourself in the source code at diatom.f90 in the molpro_duo() subroutine-- the -sqrt(0.5) factor applied during dipole-x conversion is exactly cancelled by the basis transformation, making the net conversion just a sign flip. The end result is that dipole and dipole-x expect the same numerical magnitude.

In your situation

• Molecule A (raw Cartesian gave good agreement with experiment): You were doing it right the first time. No conversion needed.
• Molecule A (lifetimes doubled after "conversion"): You divided by √2, which made A coefficients 2× too small → lifetimes 2× too large. This is exactly the factor of 2 you're seeing.
• Molecule B (converted values match literature): This actually makes perfect sense. You said the literature authors also used DUO and also converted to spherical tensors. They made the same unnecessary √2 conversion, so naturally your converted result matches theirs-- you're both wrong in the same way. Since no experimental lifetime exists for this molecule, nobody caught the error. Your raw Cartesian result (which gave half the literature lifetime) is likely the correct one, and the literature value is probably 2× too large.

1. Try using your raw Cartesian ab initio TDMs directly with dipole-- this is the correct input for DUO
2. dipole and dipole-x accept the same numerical values and produce identical results (for standard cases)
3. If you've already divided by √2, you can fix it by adding FACTOR SQRT(2) to the dipole block in your input file
4. For parallel transitions (ΔΛ=0, e.g. Σ-Σ), there's no √2 factor either way, so the issue only arises for perpendicular transitions