Completely left align number entries in a table with siunitx

I am using the siunitx package, along with the booktabs package, to make a table.

Here is my output:

Table (a) uses the tabular specification llll (i.e., no use of siunitx). Table (a) looks great, except that I want the decimal points in +0.2 and -0.4 to be lined up.

To do that, in Table (b), I used the tabular specification lSSS and used sisetup{table-number-alignment=left}. Now the decimal points are aligned, but I would like the numbers in the last two columns to be fully left aligned (flush with the k_{theta}‘s), since they are all positive. How can I do this?

Here is my code:

documentclass{article}
usepackage{siunitx,booktabs,subcaption}
DeclareSIUnit{calorie}{cal}

begin{document}

begin{table}

  begin{subtable}{textwidth}
    caption{}
    begin{tabular}{llll}
      toprule
      & {$q$ (si{elementarycharge})} & {$k_{theta}$ (si{kilocaloriepermole})} & {$k_{theta}$ (si{kilojoulepermole})}
      midrule
      compound A & +0.2 & 0.066 & 0.276144
      compound B & -0.4 & 0.14 & 0.585760
      bottomrule
    end{tabular}
  end{subtable}

  vspace*{16pt}

  begin{subtable}{textwidth}
    caption{}
    sisetup{table-number-alignment=left}
    begin{tabular}{lSSS}
      toprule
      & {$q$ (si{elementarycharge})} & {$k_{theta}$ (si{kilocaloriepermole})} & {$k_{theta}$ (si{kilojoulepermole})}
      midrule
      compound A & +0.2 & 0.066 & 0.276144
      compound B & -0.4 & 0.14 & 0.585760
      bottomrule
    end{tabular}
  end{subtable}

end{table}

end{document}

documentclass{article}
usepackage{siunitx,booktabs}
DeclareSIUnit{calorie}{cal}

begin{document}

begin{table}
    begin{tabular}{l
                    S[table-format=-1.1,table-number-alignment=left]
                    S[table-format=1.3,table-number-alignment=left]
                    S[table-format=1.6,table-number-alignment=left]}
      toprule
      & {$q$ (si{elementarycharge})} & {$k_{theta}$ (si{kilocaloriepermole})} & {$k_{theta}$ (si{kilojoulepermole})}
      midrule
      compound A & +0.2 & 0.066 & 0.276144
      compound B & -0.4 & 0.14 & 0.585760
      bottomrule
    end{tabular}

end{table}

end{document}

documentclass{article}
usepackage{siunitx,booktabs}
DeclareSIUnit{calorie}{cal}

begin{document}

begin{table}
centering

begin{tabular}{
 @{}
 l
 S[table-format=-1.1,retain-explicit-plus,table-number-alignment=left]
 S[table-format=1.3,table-number-alignment=left]
 S[table-format=1.6,table-number-alignment=left]
 @{}
}
toprule
& {$q$ (si{elementarycharge})}
& {$k_{theta}$ (si{kilocaloriepermole})}
& {$k_{theta}$ (si{kilojoulepermole})} 
midrule
compound A & +0.2 & 0.066 & 0.276144
compound B & -0.4 & 0.14 & 0.585760
bottomrule
end{tabular}

bigskip

begin{tabular}{
 @{}
 l
 S[table-format=-1.1,retain-explicit-plus]
 S[table-format=1.3]
 S[table-format=1.6]
 @{}
}
toprule
& {$q$}
& {$k_{theta}$}
& {$k_{theta}$} 
& {(si{elementarycharge})}
& {(si{kilocaloriepermole})}
& {(si{kilojoulepermole})} 
midrule
compound A & +0.2 & 0.066 & 0.276144
compound B & -0.4 & 0.14 & 0.585760
bottomrule
end{tabular}

end{table}

end{document}